Reference monitors with dynamically controlled latency

ABSTRACT

Apparatus and methods for clock synchronization and frequency translation are provided herein. Clock synchronization and frequency translation integrated circuits (ICs) generate one or more output clock signals having a controlled timing relationship with respect to one or more reference signals. The teachings herein provide a number of improvements to clock synchronization and frequency translation ICs, including, but not limited to, reduction of system clock error, reduced variation in clock propagation delay, lower latency monitoring of reference signals, precision timing distribution and recovery, extrapolation of timing events for enhanced phase-locked loop (PLL) update rate, fast PLL locking, improved reference signal phase shift detection, enhanced phase offset detection between reference signals, and/or alignment to phase information lost in decimation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/526,172, filed Jun. 28, 2017, and titled “APPARATUSAND METHODS FOR CLOCK SYNCHRONIZATION AND FREQUENCY TRANSLATION,” theentirety of which is hereby incorporated herein by reference.

FIELD OF THE DISCLOSURE

Embodiments of the invention relate to electronic devices, and moreparticularly, to circuitry for clock and signal synthesis.

BACKGROUND

A wide variety of electronic systems operate based on timing of clocksignals. For instance, examples of electronic circuitry that operatebased on clock signal timing include, but are not limited to,analog-to-digital converters (ADCs), digital-to-analog converters(DACs), wireline or optical data communication links, and/or radiofrequency front-ends.

SUMMARY OF THE DISCLOSURE

Apparatus and methods for clock synchronization and frequencytranslation are provided herein. Clock synchronization and frequencytranslation integrated circuits (ICs) generate one or more output clocksignals having a controlled timing relationship with respect to one ormore reference signals. The teachings herein provide a number ofimprovements to clock synchronization and frequency translation ICs,including, but not limited to, reduction of system clock error, reducedvariation in clock propagation delay, lower latency monitoring ofreference signals, precision timing distribution and recovery,extrapolation of timing events for enhanced phase-locked loop (PLL)update rate, fast PLL locking, improved reference signal phase shiftdetection, enhanced phase offset detection between reference signals,and/or alignment to phase information lost in decimation.

In one aspect, an integrated circuit (IC) with system clock compensationis provided. The IC includes a system clock generation circuitconfigured to generate a system clock signal based on a system referencesignal, one or more circuit blocks having timing controlled by thesystem clock signal, and a system clock compensation circuit configuredto generate one or more compensation signals operable to compensate theone or more circuit blocks for an error of the system clock signal.

In certain embodiments, the system clock compensation circuit includesan error model configured to generate an estimate of the error of thesystem clock signal based on one or more operating conditions. In anumber of embodiments, the error model is configured to receive atemperature signal indicating a temperature condition. In variousembodiments, the error model is configured to receive a vibration signalindicating a vibration condition. In several embodiments, the errormodel is configured to receive a supply voltage signal indicating asupply voltage condition. In accordance with several embodiments, the ICis configured to receive one or more coefficients of the error modelover an interface. According to some embodiments, the system clockcompensation circuit further includes a system clock error calculationcircuit configured to digitally generate the one or more compensationsignals based on the estimate from the error model. In accordance withvarious embodiments, the error model includes a polynomial model.

In some embodiments, the IC further includes a clock differencecalculation circuit configured to provide the system clock compensationcircuit with an estimate of the error of the system clock signal basedon comparing the system clock signal to a stable reference signal. Inseveral embodiments, the clock difference calculation circuit includes adigital phase-locked loop (DPLL).

In various embodiments, the system clock compensation circuit isconfigured to generate the one or more compensation signals based oncombining a closed-loop estimate of the error of the system clock signalwith an open-loop estimate of the error the system clock signal.

In a number of embodiments, the one or more circuit blocks includes atleast one of a time-to-digital converter (TDC), a filter, a DPLL, anumerically controlled oscillator (NCO), or a reference monitor.

In several embodiments, the error of the system clock signal includes atleast one of a frequency stability error or a frequency accuracy error.

In various embodiments, the system clock generation circuit includes asystem clock phased-locked loop (PLL).

In another aspect, an electronic system with system clock compensationis provided. The electronic system includes a clock source configured togenerate a system reference signal, and an IC including a systemreference pin configured to receive the system reference signal, asystem clock generation circuit configured to generate a system clocksignal based on the system reference signal, one or more circuit blockshaving timing controlled by the system clock signal, and a system clockcompensation circuit configured to generate one or more compensationsignals operable to compensate the one or more circuit blocks for anerror of the system clock signal.

In some embodiments, the system clock compensation circuit includes anerror model configured to generate an estimate of the error of thesystem clock signal based on one or more operating conditions. Accordingto several embodiments, the error model is configured to receive atemperature signal indicating a temperature condition. In a number ofembodiments, the IC includes an internal temperature sensor configuredto generate the temperature signal. In accordance with variousembodiments, the electronic system further includes an externaltemperature sensor configured to generate the temperature signal.According to a number of embodiments, the error model is configured toreceive a vibration signal indicating a vibration condition. Inaccordance with a number of embodiments, the error model is configuredto receive a supply voltage signal indicating a supply voltagecondition. According to various embodiments, the IC further includes aninterface configured to receive one or more coefficients of the errormodel. According to some embodiments, the system clock compensationcircuit further includes a system clock error calculation circuitconfigured to digitally generate the one or more compensation signalsbased on the estimate from the error model. In several embodiments, theerror model includes a polynomial model.

In some embodiments, the IC further includes a clock differencecalculation circuit configured to provide the system clock compensationcircuit with an estimate of the error of the system clock signal basedon comparing the system clock signal to a stable reference signal.According to several embodiments, the clock difference calculationcircuit includes a DPLL.

In various embodiments, the system clock compensation circuit isconfigured to generate the one or more compensation signals based oncombining a closed-loop estimate of the error of the system clock signalwith an open-loop estimate of the error the system clock signal.

In several embodiments, the one or more circuit blocks includes at leastone of a TDC, a filter, a DPLL, an NCO, or a reference monitor.

According to some embodiments, the system clock generation circuitincludes a system clock PLL.

In a number of embodiments, the clock source includes at least one of anoscillator or a resonator.

In another aspect, a method of system clock compensation is provided.The method includes generating a system clock signal based on a systemreference signal, controlling timing of one or more circuit blocks usingthe system clock signal, and digitally compensating the one or morecircuit blocks for an error of the system clock signal.

In various embodiments, the method further includes estimating the errorof the system clock signal based on one more operating conditions usinga model, and generating one or more digital compensation signals thatcontrol the one or more circuit blocks based on the estimated error.

In several embodiments, the method further includes estimating the errorof the system clock signal based on comparing the system clock signal toa stable reference signal, and generating one or more digitalcompensation signals that control the one or more circuit blocks basedon the estimated error.

According to a number of embodiments, digitally compensating the one ormore circuit blocks includes compensating at least one a TDC, a filter,a DPLL, an NCO, or a reference monitor.

In another aspect, an electronic system includes an IC including atiming circuit configured to generate an output signal based on timingof an input reference signal, an output pin configured to receive theoutput signal from the timing circuit, and a delay compensation circuitconfigured to provide one or more compensation signals to the timingcircuit. The electronic system further includes a signal path configuredto route the output signal from the output pin to a destination node.The one or more compensation signals are operable to digitallycompensate the timing circuit for a variation in delay of the signalpath.

In some embodiments, the delay compensation circuit includes a delaymodel configured to generate an estimate of the variation in delay basedon one or more operating conditions. In various embodiments, the delaymodel is configured to receive a temperature signal indicating atemperature condition. According to a number of embodiments, the ICfurther includes an interface configured to receive one or morecoefficients of the delay model. In several embodiments, the delaycompensation circuit further includes a delay error calculation circuitconfigured to digitally generate the one or more compensation signalsbased on the estimate from the delay model. In accordance with certainembodiments, the delay model includes a polynomial model. According tovarious embodiments, the delay model is further configured to accountfor an internal delay of the IC.

In a number of embodiments, the electronic system further includes areturn path of the output signal, and the IC further includes a returnpath pin configured to receive a returned signal from the return path,and a delay difference detector configured to provide the delaycompensation circuit with an estimate of the delay of the signal pathbased on comparing the output signal to the returned signal. In variousembodiments, the IC further includes a delay error calculation circuitconfigured to generate the one or more compensation signals based onaccounting for a round trip delay of the output signal from the outputpin to the return pin.

In several embodiments, the timing circuit includes a DPLL. In accordingwith certain embodiments, at least one of the one or more compensationsignals is configured to provide a digital adjustment to the DPLL.

In various embodiments, the timing circuit includes at least onedigitally-controllable delay element configured to receive at least oneof the compensation signals.

In another aspect, an IC with compensation for signal path delayvariation is provided. The IC includes a timing circuit configured togenerate an output signal based on timing of an input reference signal,an output pin configured to provide the output signal to a destinationnode via a signal path, and a delay compensation circuit configured togenerate one or more compensation signals operable to digitallycompensate the timing circuit for a variation in delay of the signalpath to thereby control a phase of the output signal at the destinationnode relative to a phase of the input reference signal.

In some embodiments, the delay compensation circuit includes a delaymodel configured to generate an estimate of the variation in delay basedon one or more operating conditions. In accordance with certainembodiments, the delay model is configured to receive a temperaturesignal indicating a temperature condition. In various embodiments, theIC further includes an interface configured to receive one or morecoefficients of the delay model. In several embodiments, the delaycompensation circuit further includes a delay error calculation circuitconfigured to digitally generate the one or more compensation signalsbased on the estimate from the delay model. In a number of embodiments,the delay model includes a polynomial model. According to variousembodiments, the delay model is further configured to account for aninternal delay of the IC.

In certain embodiments, the IC further includes a return path pinconfigured to receive a returned signal from the signal path, and adelay difference detector configured to provide the delay compensationcircuit with an estimate of the delay of the signal path based oncomparing the output signal to the returned signal. According to variousembodiments, the IC further includes a delay error calculation circuitconfigured to generate the one or more compensation signals based onaccounting for a round trip delay of the output signal from the outputpin to the return pin.

In several embodiments, the timing circuit includes a DPLL. According tovarious embodiments, at least one of the one or more compensationsignals is configured to provide a digital adjustment to the DPLL.

In a number of embodiments, the timing circuit includes at least onedigitally-controllable delay element configured to receive at least oneof the compensation signals.

In another aspect, a method of signal path delay compensation in anelectronic system is provided. The method includes generating an outputsignal based on an input reference signal using a timing circuit of anIC, providing the output signal from an output pin of the IC to adestination node via a signal path, and digitally compensating thetiming circuit for variation in delay of the signal path to therebycontrol a phase of the output signal at the destination node relative toa phase of the input reference signal.

In a number of embodiments, the method further includes estimating thevariation in delay based on one more operating conditions using a delaymodel, and generating one or more digital compensation signals fordigitally compensating the timing circuit based on the estimated error.

In several embodiments, the method further includes receiving a returnsignal on a return signal pin of the IC, estimating the variation indelay based on comparing the output signal to the return signal, andgenerating one or more digital compensation signals for digitallycompensating the timing circuit based on the estimated error.

In some embodiments, digitally compensating the timing circuit includesproviding a phase adjustment to a DPLL.

In another aspect, an IC with compensation for signal path delayvariation is provided. The IC includes a timing circuit configured togenerate an output signal based on timing of an input reference signal,a signal path configured to provide the output signal to a destinationnode, and a delay compensation circuit configured to generate one ormore compensation signals operable to digitally compensate the timingcircuit for a variation in delay of the signal path to thereby control aphase of the output signal at the destination node relative to a phaseof the input reference signal.

In certain embodiments, the delay compensation circuit includes a delaymodel configured to generate an estimate of the variation in delay basedon one or more operating conditions. In a number of embodiments, thedelay model is configured to receive a temperature signal indicating atemperature condition. According to several embodiments, the IC furtherincludes an interface configured to receive one or more coefficients ofthe delay model. In some embodiments, the delay compensation circuitfurther includes a delay error calculation circuit configured todigitally generate the one or more compensation signals based on theestimate from the delay model. According to various embodiments, thedelay model includes a polynomial model. In accordance with severalembodiments, the delay model is further configured to account for aninternal delay of the IC.

In a number of embodiments, the timing circuit includes a DPLL.According to several embodiments, at least one of the one or morecompensation signals is configured to provide a digital adjustment tothe DPLL.

In various embodiments, the timing circuit includes at least onedigitally-controllable delay element configured to receive at least oneof the compensation signals.

In another aspect, an IC with reference monitoring is provided. The ICincludes a clock measurement circuit configured to generate a pluralityof digital measurements of a reference clock signal based on timing of asystem clock signal, and a reference monitor configured to generate amonitor output signal indicating whether the reference clock signal iswithin a tolerance of one or more tolerance parameters. The referencemonitor includes a statistical processing circuit configured to processthe plurality of digital measurements to generate an estimate ofmeasurement uncertainty, and to control a latency of the referencemonitor in generating the monitor output signal based on the estimate ofmeasurement uncertainty.

In some embodiments, the statistical processing circuit is configured tocompute a variance of the plurality of digital measurements over a timewindow. According to a number of embodiments, the one or more toleranceparameters includes a nominal period and a period offset limit, and thestatistical processing circuit is further configured to control thelatency based on comparing the variance to the period offset limit.

In a number of embodiments, the statistical processing circuit isfurther configured to determine a number of samples of the referenceclock signal sufficient to estimate a period of the reference clocksignal within a confidence interval.

In various embodiments, the one or more tolerance parameters includes ajitter limit.

According to a number of embodiments, the statistical processing circuitis further configured to generate a plurality of estimates ofmeasurement uncertainty associated with a plurality of partiallyoverlapping time windows.

In several embodiments, the statistical processing circuit is configuredto compute a mean and a variance of the plurality of digitalmeasurements over a time window.

In some embodiments, the clock measurement circuit includes a TDCconfigured to generate a plurality of digital time stamps representing aplurality of transition times of the reference clock signal. Accordingto various embodiments, the IC further includes a DPLL configured toprocess the plurality of digital time stamps.

In another aspect, a method of reference monitoring in a clock system isprovided. The method includes generating a plurality of digitalmeasurements of a reference clock signal based on timing of a systemclock signal, processing the plurality of digital measurements togenerate an estimate of measurement uncertainty using a referencemonitor, and controlling a measurement latency of the reference monitorbased on the estimate of measurement uncertainty.

In various embodiments, the method further includes detecting whetherthe reference clock signal is within a tolerance of one or moretolerance parameters using the reference monitor.

In several embodiments, processing the plurality of digital measurementsincludes computing a variance of the plurality of digital measurementsover a time window.

In a number of embodiments, processing the plurality of digitalmeasurements includes determining a number of samples of the referenceclock signal sufficient to estimate a period of the reference clocksignal within a confidence interval.

In various embodiments, generating the plurality of digital measurementsincludes generating a plurality of digital time stamps representing aplurality of transition times of the reference clock signal.

In another aspect, a reference signal monitoring system with dynamicallycontrolled latency is provided. The reference signal monitoring systemincludes a TDC configured to generate a plurality of digital time stampsrepresenting a plurality of transition times of a reference clocksignal, and a reference monitor configured to generate a monitor outputsignal indicating a status of the reference clock signal. The referencemonitor is configured to process the plurality of digital time stamps togenerate an estimate of measurement uncertainty, and to control alatency of the reference monitor in generating the monitor output signalbased on the estimate of measurement uncertainty.

In a number of embodiments, the reference monitor is further configuredto compute a variance of the plurality of digital time stamps over atime window. According to various embodiments, the reference monitor isfurther configured to control the latency based on comparing thevariance to a period offset limit.

In several embodiments, the reference monitor is further configured todetermine a number of samples of the reference clock signal sufficientto estimate a period of the reference clock signal within a confidenceinterval.

In various embodiments, the monitor output signal indicates whether thereference clock signal is within a jitter limit.

According to a number of embodiments, the reference monitor is furtherconfigured to generate a plurality of estimates of measurementuncertainty associated with a plurality of partially overlapping timewindows.

In another aspect, a distributed timing system is provided. Thedistributed timing system includes a source IC configured to detect atiming of a signal based on a common reference signal, and to generate adigital timing signal that digitally represents the timing of thesignal. The distributed timing system further includes a digitalinterface electrically coupled to the source IC, and a destination ICconfigured to receive the digital timing signal from the digitalinterface. The destination IC is configured to recover the signal basedon the digital timing signal and the common reference signal.

In various embodiments, the source IC includes a TDC configured togenerate a plurality of digital time stamps representing a plurality oftransition times of the signal, and a format conversion circuitconfigured to generate the digital timing signal based on the pluralityof digital time stamps. According to a number of embodiments, the sourceIC further includes a synchronization circuit configured to synchronizethe TDC and the format conversion circuit based on the common referencesignal. In several embodiments, the distributed timing system furtherincludes a system clock PLL configured to generate a system clock signalfor the synchronization circuit based on a local system referencesignal.

In some embodiments, the digital interface is a serial interface.

In several embodiments, the distributed timing system further includesone or more additional source ICs configured to provide one or moreadditional digital timing signals to the digital interface.

In a number of embodiments, the destination IC includes a formatconversion circuit configured to process the digital timing signal togenerate a plurality of digital time stamps representing a plurality oftransition times of the signal. According to certain embodiments, thedistributed timing system further includes a DPLL configured to recoverthe signal based on the plurality of digital time stamps. In severalembodiments, the source IC further includes a synchronization circuitconfigured to synchronize the format conversion circuit based on thecommon reference signal. According to some embodiments, the distributedtiming system further includes a system clock PLL configured to generatea system clock signal for the synchronization circuit based on a localsystem reference signal.

In certain embodiments, the distributed timing system further includesone or more additional destination ICs configured to receive the digitaltiming signal from the timing interface, and to recover the signal basedon the digital timing signal and the common reference signal.

In a number of embodiments, the destination IC recovers a frequency ofthe signal.

In various embodiments, the destination IC recovers both a frequency ofthe signal and a phase of the signal.

In another aspect, a clock synchronization and frequency translation ICis provided. The IC includes a first pin configured to receive a digitaltiming signal representing a timing of a signal, a format conversioncircuit configured to process the digital timing signal to generate aplurality of reference digital time stamps indicating a plurality oftransition times of the signal, and a DPLL configured to recover thesignal from the plurality of reference digital time stamps.

In a number of embodiments, the DPLL recovers a frequency of the signal.

In various embodiments, the DPLL recovers both a frequency of the signaland a phase of the signal.

In some embodiments, the IC further includes a second pin configured toreceive a common reference signal, and a synchronization circuitconfigured to synchronize the format conversion circuit based on thecommon reference signal. According to several embodiments, the ICfurther includes a third pin configured to receive a system referencesignal, and a system clock PLL configured to generate a system clocksignal for the synchronization circuit based on a system referencesignal.

In another aspect, a method of distributed timing is provided. Themethod includes detecting timing of a signal based on a common referencesignal using a first IC, generating a digital representation of thedetected timing using the first IC, transmitting the digitalrepresentation of the detected timing from the first IC to a second ICover a digital interface, and recovering the signal in the second ICbased on the digital representation of the detected timing and thecommon reference signal.

In various embodiments, generating the digital representation of thedetected timing includes using a TDC to generate a plurality of digitaltime stamps representing a plurality of transition times of the signal.

According to a number of embodiments, recovering the signal in thesecond IC includes processing the digital representation of the detectedtiming to generate a plurality of reference digital time stampsrepresenting a plurality of transition times of the signal. In certainembodiments, recovering the signal in the second IC further includesusing a DPLL to recover the signal from the plurality of referencedigital time stamps.

In several embodiments, recovering the signal in the second IC includesrecovering both a frequency of the signal and a phase of the signal.

In another aspect, a distributed timing system is provided. Thedistributed timing system includes a source device configured receive acommon time-base signal and to generate a digital data signalrepresenting a timing of a signal. The distributed timing system furtherincludes a data hub configured to receive the digital data signal, and adestination device configured to receive the digital data signal fromthe data hub, and to recover the signal based on the common time-basesignal and the digital data signal.

In a number of embodiments, the distributed timing system furtherincludes one or more additional destination devices configured toreceive the digital data signal from the data hub, and to recover thesignal based on the common time-base signal and the digital data signal.

In several embodiments, the distributed timing system further includesone or more additional source devices configured to generate one or moredigital data signals representing timing of one or more signals, and toprovide the one or more digital data signals to the data hub.

In various embodiments, the source device is configured to receive afirst local oscillator signal that controls local timing at the sourcedevice, and the destination device is configured to receive a secondlocal oscillator signal that controls timing at the destination device.

In a number of embodiments, the destination device recovers a frequencyof the signal.

In several embodiments, the destination device recovers both a frequencyof the signal and a phase of the signal.

In another aspect, a method of phase detection in a DPLL is provided.The method includes generating a digital representation of a firsttiming event of an input clock signal to a phase detector, generating adigital representation of a second timing event of the input clocksignal, extrapolating a first extrapolated timing event based onadjusting the digital representation of the second timing event by atime interval between the second timing event and the first timingevent, and providing phase detection using the first extrapolated timingevent.

In various embodiments, the input clock signal includes a referenceclock signal to the DPLL.

In a number of embodiments, the input clock signal includes a feedbackclock signal to the DPLL.

In several embodiments, extrapolating the first extrapolated timingevent includes backwards extrapolation.

In some embodiments, extrapolating the first extrapolated timing eventincludes forward extrapolation.

In various embodiments, the method further includes using a TDC togenerate the digital representations of the first and second timingevents.

In certain embodiments, the method further includes estimating the timeinterval from the input clock signal.

In accordance with a number of embodiment, the method further includesdetermining the time interval based on an ideal periodicity of thetiming events of the input clock signal.

In several embodiments, the method further includes generating a digitalrepresentation of a third timing event of the input clock signal, andextrapolating a second extrapolated timing event based on adjusting thedigital representation of the third timing event by a time intervalbetween the third timing event and the first timing event.

In some embodiments, the first timing event corresponds to an edgeassociated with a carrier frequency of the input clock signal, and thesecond timing event corresponds to an edge associated with a sub-carrierfrequency of the input clock signal.

In several embodiments, the first timing event conveys phase informationof the input clock signal, and the second timing event conveys frequencyinformation of the input clock signal.

In another aspect, a DPLL includes a first timing detector configured togenerate a first plurality of digital representations of timing of afirst clock signal, and the first plurality of digital representationsinclude a first digital representation of a first timing event and asecond digital representation of a second timing event. The DPLL furtherincludes a second timing detector configured to generate a secondplurality of digital representations of timing of a second clock signal,and a phase detector configured to provide phase detection based on thefirst plurality of digital representations and the second plurality ofdigital representations. The phase detector is configured to generate afirst extrapolated timing event based on adjusting the second digitalrepresentation by a time interval between the second timing event andthe first timing event, and the phase detector is configured to providephase detection based on the first extrapolated timing event.

In a number of embodiments, the first clock signal is a reference clocksignal to the DPLL, and the second clock signal is a feedback clocksignal to the DPLL.

In various embodiments, the first clock signal is a feedback clocksignal to the DPLL, and the second clock signal is a reference clocksignal to the DPLL.

In several embodiments, the phase detector is configured to generate thefirst extrapolated timing event based on backwards extrapolation.

In some embodiments, the phase detector is configured to generate thefirst extrapolated timing event based on forward extrapolation.

According to certain embodiments, the first timing detector includes afirst TDC and the second timing detector includes a second TDC.

In a number of embodiments, the phase detector is configured to estimatethe time interval based on the first plurality of digitalrepresentations and the second plurality of digital representations.

In several embodiments, the phase detector is configured to determinethe time interval based on an ideal periodicity of the first clocksignal.

According to some embodiments, the first plurality of digitalrepresentations includes a third digital representation of a thirdtiming event, and the phase detector is further configured to generate asecond extrapolated timing event based on adjusting the digitalrepresentation of the third timing event by a time interval between thethird timing event and the first timing event.

In a number of embodiments, the first timing event corresponds to anedge associated with a carrier frequency of the first clock signal, andthe second timing event corresponds to an edge associated with asub-carrier frequency of the first clock signal.

In accordance with various embodiments, the first timing event conveysphase information of the first clock signal, and the second timing eventconveys frequency information of the first clock signal.

In another aspect, a method of locking frequency and phase with highspeed is provided. The method includes detecting a frequency offsetbetween a reference signal and a feedback signal of a PLL, compensatingfor the frequency offset by providing a frequency offset correction tothe PLL with a feedback loop of the PLL opened, compensating for a phaseoffset between the reference signal and the feedback signal by providinga phase offset correction after the frequency offset correction, andcompensating for a residual error of the PLL by locking the feedbacksignal to the reference signal with the feedback loop of the PLL closed.

In a number of embodiments, detecting the frequency offset includessubtracting an initial phase offset from an output of a digital phasedetector, and detecting the frequency offset based on a residual phaseoffset of the digital phase detector.

In several embodiments, compensating for the frequency offset includescontrolling a loop filter output value.

In some embodiments, detecting the frequency offset includes comparing aderivative of successive phase measurements of the reference clocksignal to a derivate of successive phase measurements of the feedbackclock signal. According to certain embodiments, the method furtherincludes calculating a fractional frequency error based on thecomparison. In various embodiments, compensating for the frequencyoffset includes normalizing the fractional frequency error by a controlword of an NCO, and updating the NCO based on the normalized frequencyerror. In accordance with several embodiments, compensating for thefrequency offset includes gradually transitioning an output frequency ofthe PLL with a controlled rate of change.

In a number of embodiments, compensating for the phase offset includessynchronizing a feedback divider of the PLL based on timing of thereference clock signal.

In some embodiments, compensating for the phase offset includesgradually providing phase adjustment to limit an output frequencydeviation of the PLL.

In accordance with certain embodiments, detecting the frequency offsetincludes detecting the frequency offset using a reference monitor.

In several embodiments, compensating for the phase offset includesproviding an open loop phase correction to the PLL.

In a number of embodiments, compensating for the phase offset includesproviding a closed loop phase correction to the PLL.

In various embodiments, compensating for the residual error of the PLLincludes decreasing a loop bandwidth of the PLL over time.

In another aspect, an IC providing locking of frequency and phase withhigh speed is provided. The IC includes a DPLL including a digital phasedetector configured to compare a reference signal and a feedback signal.The IC further includes a frequency offset detection circuit configuredto detect a frequency offset between the reference signal and thefeedback signal, and a loop controller configured to provide a frequencyoffset correction to the DPLL with a feedback loop of the DPLL opened.The loop controller is further configured to compensate for a phaseoffset between the reference signal and the feedback signal by providinga phase offset correction after the frequency offset correction, and tocompensate for a residual error of the DPLL by locking the feedbacksignal to the reference signal with the feedback loop of the DPLLclosed.

In a number of embodiments, the frequency offset detection circuit isconfigured to detect the frequency offset by subtracting an initialphase offset from an output of the digital phase detector, and to detectthe frequency offset based on a residual phase offset of the digitalphase detector.

In several embodiments, the loop controller is configured to provide thefrequency offset correction based on controlling a loop filter outputvalue of a loop filter of the DPLL.

In certain embodiments, the frequency offset detection circuit isconfigured to detect the frequency offset by comparing a derivative ofsuccessive phase measurements of the reference clock signal to aderivate of successive phase measurements of the feedback clock signal.

In various embodiments, the loop controller is configured to compensatefor the frequency offset based on normalizing a fractional frequencyerror by a control word of an NCO, and updating the NCO based on thenormalized frequency error.

In a number of embodiments, the loop controller is further configured togradually transition an output frequency of the DPLL with a controlledrate of change.

In several embodiments, the loop controller is further configured tosynchronize a feedback divider of the DPLL based on timing of thereference clock signal.

In some embodiments, the loop controller is configured to compensate forthe residual error of the DPLL based on decreasing a loop bandwidth ofthe DPLL over time.

In another aspect, an IC with reference phase shift detection isprovided. The IC includes a phase shift detector configured to detect aphase shift of a reference signal based on timing of a system clocksignal. The phase shift detector is configured generate a phasedetection signal indicating the detected phase shift based on observingthe reference signal over a plurality of cycles of the system clocksignal.

In a number of embodiments, the phase shift detector includes a phaseerror differentiation circuit configured to differentiate a phase errorobserved on one or more cycles of the plurality of cycles.

In several embodiments, the phase shift detector includes a windowedaccumulation circuit configured to accumulate two or more observationsof the reference signal captured over the plurality of cycles.

In some embodiments, the phase shift detector includes a majorityprocessing circuit configured to generate the phase detection cyclebased on majority processing of two or more observations of thereference signal captured over the plurality of cycles.

In a number of embodiments, the IC further includes a PLL configured toreceive the reference signal.

In another aspect, an IC with phase offset detection is provided. The ICincludes a phase offset detector configured to detect a phase offsetbetween a first reference signal and a second reference signal. Thephase offset detector is configured generate a phase offset signalindicating the detected phase offset based on observing a phasedifference between the first reference signal and the second referencesignal over a plurality of cycles of a system clock signal.

In several embodiments, the IC further includes a PLL configured toreceive at least one of the first reference signal or the secondreference signal. According to some embodiments, the IC further includesa multiplexer configured to provide the first reference signal or thesecond reference signal. In accordance with various embodiments, thephase offset detector is configured to provide a phase adjustment to thePLL using the phase offset signal.

In certain embodiments, the phase offset detector receives a firstplurality of digital representations of timing of the first referencesignal, and a second plurality of digital representations of the secondreference signal.

In a number of embodiments, the IC further includes a first TDCconfigured to generate the first plurality of digital representations,and a second TDC configured to generate the second plurality of digitalrepresentations.

In another aspect, a DPLL is provided. The DPLL includes a dividerconfigured to divider an input clock signal to generate a divided clocksignal having a first timing event and a second timing event. The DPLLfurther includes a phase detector configured to provide phase detectionbased on the divided clock signal. The phase detector includes aninterpolation circuit configured to generate an interpolated timingevent between the first timing event and the second timing event, andthe phase detector is configured to provide phase detection based on theinterpolated timing event.

In some embodiments, the interpolated timing event corresponds to atiming event lost by decimation of the input clock signal by thedivider.

In several embodiments, the input clock signal is a reference clocksignal to the DPLL.

In certain embodiments, the input clock signal is a feedback clocksignal to the DPLL.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a clocksynchronization and frequency translation integrated circuit (IC).

FIG. 2A is a schematic diagram of one implementation of a digitalphase-locked loop (DPLL) for a clock synchronization and frequencytranslation IC.

FIG. 2B is a schematic diagram of one implementation of an analogphase-locked loop (APLL) for a clock synchronization and frequencytranslation IC.

FIG. 2C is a schematic diagram of one implementation of a system clockphase-locked loop (PLL) for a clock synchronization and frequencytranslation IC.

FIG. 3 is a schematic diagram of another implementation of a DPLL for aclock synchronization and frequency translation IC.

FIG. 4 is a schematic diagram of one implementation of a numericallycontrolled oscillator (NCO) for a clock synchronization and frequencytranslation IC.

FIG. 5 is a schematic diagram of one implementation of frequencytranslation loops for a clock synchronization and frequency translationIC.

FIG. 6 is a schematic diagram of one embodiment of an electronic systemwith system clock compensation.

FIG. 7 is a schematic diagram of another embodiment of an electronicsystem with system clock compensation.

FIG. 8 is a schematic diagram of another embodiment of an electronicsystem with system clock compensation.

FIG. 9 is a schematic diagram of another embodiment of an electronicsystem with open loop system clock compensation.

FIG. 10 is a schematic diagram of an IC with closed loop system clockcompensation.

FIG. 11 is a schematic diagram of a system clock compensation circuitaccording to another embodiment.

FIG. 12 is a schematic diagram of a TDC according to one embodiment.

FIG. 13 is a schematic diagram of a DPLL according to anotherembodiment.

FIG. 14 is a schematic diagram of an NCO according to anotherembodiment.

FIG. 15 is a schematic diagram of one embodiment of an electronic systemwith delay compensation.

FIG. 16 is a schematic diagram of another embodiment of an electronicsystem with delay compensation.

FIG. 17 is a schematic diagram of another embodiment of an electronicsystem with delay compensation.

FIG. 18 is a schematic diagram of another embodiment of a clocksynchronization and frequency translation IC.

FIG. 19 is a schematic diagram of another embodiment of an IC with delaycompensation.

FIG. 20 is a schematic diagram of another embodiment of an electronicsystem with delay compensation.

FIG. 21 is a schematic diagram of one embodiment of a referencemonitoring system.

FIG. 22 is a schematic diagram of another embodiment of a referencemonitoring system.

FIG. 23 is a schematic diagram of another embodiment of a referencemonitoring system.

FIG. 24 is a schematic diagram of an electronic system according toanother embodiment.

FIG. 25 is a schematic diagram of an electronic system according toanother embodiment.

FIG. 26A is a schematic diagram of a source device according to oneembodiment.

FIG. 26B is a schematic diagram of a destination device according to oneembodiment.

FIG. 27A is a schematic diagram of a source IC according to oneembodiment.

FIG. 27B is a schematic diagram of a destination IC according to oneembodiment.

FIG. 28 is a schematic diagram of another embodiment of a clocksynchronization and frequency translation IC.

FIG. 29 is a schematically depicts various timing event sequences forone example of intermediate decimation.

FIG. 30A illustrates one example of a backward extrapolation of asequence of timing events.

FIG. 30B illustrates one example of forward and backward extrapolationof a sequence of timing events.

FIG. 31 is a schematic diagram of a DPLL according to anotherembodiment.

FIG. 32 is a schematic diagram of a DPLL according to anotherembodiment.

FIG. 33 is a schematic diagram of another implementation of frequencytranslation loops for a clock synchronization and frequency translationIC.

FIG. 34 is a method of phase and frequency locking according to oneembodiment.

FIGS. 35A-35E illustrate various embodiments of DPLL circuitry for phaseand frequency locking.

FIGS. 36A-36D are graphs of various example of phase step detection.

FIGS. 37A-37D are schematic diagrams of various embodiments of phaseshift detectors.

FIG. 38 is a schematic diagram of a phase offset detection systemaccording to one embodiment.

FIG. 39 is a graph of one example of possible phases after a divide bythree.

FIG. 40 is a graph of one example of time stamp interpolations.

FIG. 41 is a schematic diagram of a DPLL according to anotherembodiment.

FIG. 42 is a schematic diagram of a DPLL according to anotherembodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Various aspects of the novel systems, apparatuses, and methods aredescribed more fully hereinafter with reference to the accompanyingdrawings. Aspects of this disclosure may, however, be embodied in manydifferent forms and should not be construed as limited to any specificstructure or function presented throughout this disclosure. Rather,these aspects are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the disclosure to thoseskilled in the art.

Based on the teachings herein, one skilled in the art should appreciatethat the scope of the disclosure is intended to cover any aspect of thenovel systems, apparatuses, and methods disclosed herein, whetherimplemented independently of or combined with any other aspect. Forexample, an apparatus may be implemented or a method may be practicedusing any number of the aspects set forth herein. Thus, it will beunderstood that certain embodiments can include more elements thanillustrated in a drawing and/or a subset of the elements illustrated ina drawing. Further, some embodiments can incorporate any suitablecombination of features from two or more drawings. In addition, thescope is intended to encompass such an apparatus or method which ispracticed using other structure, functionality, or structure andfunctionality in addition to or other than the various aspects set forthherein. It should be understood that any aspect disclosed herein may beembodied by one or more elements of a claim or equivalent thereof.

Although particular aspects are described herein, many variations andpermutations of these aspects fall within the scope of the disclosure.Although some benefits and advantages of the preferred aspects arementioned, the scope of the disclosure is not intended to be limited toparticular benefits, uses, or objectives. Rather, aspects of thedisclosure are intended to be broadly applicable to a variety ofelectronic systems. The detailed description and drawings are merelyillustrative of the disclosure rather than limiting, the scope of thedisclosure being defined by the appended claims and equivalents thereof.

FIG. 1 is a schematic diagram of one embodiment of a clocksynchronization and frequency translation integrated circuit (IC) 40.The clock synchronization and frequency translation IC 40 illustratesone embodiment of an IC that can be implemented in accordance with oneor more features of the present disclosure. However, the teachingsherein are applicable to other implementations of electronic systems,including, but not limited to, other implementations of ICs. An IC isalso referred to herein as a semiconductor chip or semiconductor die.

In the illustrated embodiment, the clock synchronization and frequencytranslation IC 40 includes a first input reference control circuit 1 a,a second input reference control circuit 1 b, a first reference clockdemodulator 2 a, a second reference clock demodulator 2 b, a thirdreference clock demodulator 2 c, a fourth reference clock demodulator 2d, a first reference divider 3 a, a second reference divider 3 b, athird reference divider 3 c, a fourth reference divider 3 d, a firsttime-to-digital converter (TDC) 4 a, a second TDC 4 b, a third TDC 4 c,a fourth TDC 4 d, a digital cross point multiplexer 5, a first digitalphase-locked loop (DPLL) 6 a, a second DPLL 6 b, a first analogphase-locked loop (APLL) 7 a, a second APLL 7 b, a first output clockmultiplexer 8 a, a second output clock multiplexer 8 b, a third outputclock multiplexer 8 c, a fourth output clock multiplexer 8 d, a fifthoutput clock multiplexer 8 e, a first output divider 9 a, a secondoutput divider 9 b, a third output divider 9 c, a fourth output divider9 d, a fifth output divider 9 e, a first clock output driver 11 a, asecond clock output driver 11 b, a third clock output driver 11 c, afourth clock output driver 11 d, a fifth clock output driver 11 e, afirst feedback clock multiplexer 12 a, a second feedback clockmultiplexer 12 b, a third feedback clock multiplexer 12 c, a fourthfeedback clock multiplexer 12 d, a fifth feedback clock multiplexer 12e, a system clock PLL 13, a modulation and phase offset controller 14, atemperature sensor 15, a system clock compensation circuit 16, aninternal zero delay control circuit 17, reference monitors 18, areference switching circuit 19, auxiliary numerically controlledoscillators (NCOs) 21, auxiliary TDCs 22, a status and control pinsinterface 23, and a serial port and memory controller 24.

As shown in FIG. 1, the clock synchronization and frequency translationIC 40 further includes various pins or pads, including input referencepins (REFA, REFAA, REFB, REFBB), system reference pins (XOA, XOB),output clock pins (OUT0AP, OUT0AN, OUT0BP, OUT0BN, OUT0CP, OUT0CN,OUT1AP, OUT1AN, OUT1BP, OUT1BN), serial port pins (SERIAL PORT), andmultifunction pins (M PINS). For clarity of the figures, certain pinshave been omitted from FIG. 1, such as pins used for power and ground.

Although one example of circuitry and pins is shown for a clocksynchronization and frequency translation chip, other implementationsand circuitry and/or pins can be used.

The input reference pins (REFA, REFAA, REFB, REFBB) receive inputreference signals (for instance reference clock signals or other phaseand/or frequency reference signals), which are handled by the inputreference control circuits 1 a-1 b. The input reference control circuits1 a-1 b can be used to provide input reference selection, inversion,and/or a wide variety of other processing. In certain implementations,the input reference control circuits 1 a-1 b are configurable to processeither differential or singled-ended input reference signals, therebyenhancing the flexibility of the IC 40.

In certain implementations, one or more of the input reference pins(REFA, REFAA, REFB, REFBB) receives a reference clock signal having acarrier frequency and an embedded subcarrier frequency. Such a referenceclock signal includes a low frequency clock signal embedded within ahigh frequency carrier.

Providing a reference clock signal with an embedded subcarrier canprovide a number of advantages. For example, in applications including achassis with timing cards and line cards, the carrier frequency canconvey desired frequency information while the subcarrier frequency canconvey desired phase information.

The reference demodulators 2 a-2 d serve to extract phase informationassociated with a subcarrier frequency of a reference clock signal. Forexample, when enabled, a reference demodulator recovers modulationevents corresponding to periodic phase variations appearing on certainedges of the input reference clock signal. A reference clock signal withan embedded subcarrier frequency can be generated in a variety of ways,including, but not limited to, by another instantiation of a clocksynchronization and frequency translation IC.

Accordingly, the reference demodulators 2 a-2 d serve to extractmodulation events embedded in a received input reference clock signal inapplications including an embedded subcarrier frequency.

The reference dividers 3 a-3 d operate to provide division to acorresponding input reference signal. In the illustrated embodiment, thereference dividers 3 a-3 d operate using programmable divisor values.For instance, desired divisor values can be programmed into the IC 40 bya user via the serial port. By including the reference dividers 3 a-3 d,flexibility of the chip is enhanced by providing control over thefrequencies of input reference signals. For example, the referencedividers 3 a-3 d can be used to reduce the reference frequencies tovalues suitable for the input frequency range of the TDCs 4 a-4 d.

The TDCs 4 a-4 d provide time-to-digital conversion of the dividedreference signals from the reference dividers 3 a-3 d, respectively. Inparticular, each of the TDCs 4 a-4 d operates to observe the timing of acorresponding reference signal, and to generate digital time stampsidentifying when edge transitions (for instance, rising and/or fallingedges) of the reference signal occur.

The digital cross point multiplexer 5 operates to route various signalsthroughout the IC 40 as desired. Although certain inputs and outputs areillustrated in FIG. 1, the digital cross point multiplexer 5 can beadapted to route a wide variety of signals throughout the IC 40.Furthermore, the digital cross point multiplexer 5 is also connected tovarious pins and interfaces of the IC 40, and thus can be used, forexample, to send or receive signals via the serial port pins (SERIALPORT) and/or multifunction pins (M PINS).

The digital cross point multiplexer 5 is digitally programmable toprovide connectivity desired for a particular application orimplementation. In a first example, the digital cross point multiplexer5 provides digital time stamps from the output of one or more of theTDCs 4 a-4 d to the first DPLL 6 a and/or the second DPLL 6 b. In asecond example, the digital cross point multiplexer 5 provides digitaltime stamps from the output of one or more of the TDCs 4 a-4 d to thereference monitors 18. In a third example, the digital cross pointmultiplexer 5 connects the auxiliary NCOs 21 and/or auxiliary TDCs 22 tothe DPLLs 6 a, 6 b and/or other circuitry of the IC 40.

With continuing reference to FIG. 1, the first DPLL 6 a processesdigital time stamps received from the digital cross point multiplexer 5to generate a first DPLL output clock signal, which serves as an inputto the first APLL 7 a. Additionally, the APLL 7 a provides frequencytranslation and/or jitter cleanup to generate a first APLL output clocksignal. Similarly, the second DPLL 6 b processes digital time stampsreceived from the digital cross point multiplexer 5 to generate a secondDPLL output clock signal, which the second APLL 7 b uses as a referencefor generating a second APLL output clock signal.

The output clock multiplexers 8 a-8 e are used for selecting anddistributing output clock signals from the first APLL 7 a, the secondAPLL 7 b, and/or the system clock PLL 13 to the output dividers 9 a-9 e.The output dividers 9 a-9 e provide programmable division to the outputclock signals chosen by the output clock multiplexers 8 a-8 e,respectively. In the illustrated embodiment, the output clock dividers 9a-9 e also operate with a controllable phase delay and/or burst controlto support a burst clocking specification, such as JESD204B. The outputdividers 9 a-9 e also support modulation of the location of edges of theoutput clocks signals (for instance, rising or falling edges) to supportinsertion of a subcarrier into a higher frequency carrier clock signal.Thus, the output dividers 9 a-9 e can also be used to generate a clocksignal with an embedded subcarrier frequency, which can be demodulatedby a reference demodulator (for instance, reference demodulators 2 a-2d) of another instantiation of the IC 40.

The divided output clock signals from the dividers 9 a-9 e are providedto the clock output drivers 11 a-11 e, respectively, which drive theoutput clock pins (OUT0AP, OUT0AN, OUT0BP, OUT0BN, OUT0CP, OUT0CN,OUT1AP, OUT1AN, OUT1BP, OUT1BN). Furthermore, the divided output clocksignals are also provided to the feedback clock multiplexers 12 a-12 e,which can be used to provide one or more selected clock signals to theinternal zero delay control circuit 17 and/or other clock feedbackpath(s).

As shown in FIG. 1, the internal zero delay control circuit 17 isconnectable via the digital cross point multiplexer 5 to the DPLLs 6 a,6 b and/or other circuitry and/or pins of the IC 40. The internal zerodelay control circuit 17 aids in controlling an output phase at theoutput clock pins (OUT0AP, OUT0AN, OUT0BP, OUT0BN, OUT0CP, OUT0CN,OUT1AP, OUT1AN, OUT1BP, OUT1BN) relative to an input phase of the inputreference signals received on the input reference pins (REFA, REFAA,REFB, REFBB). For example, the internal zero delay control circuit 17can be used to operate the IC 40 as a PLL with about zero degrees ofphase delay between the input phase and the output phase.

The system clock PLL 13 receives one or more system reference signalsfrom the system reference pins (XOA, XOB). Additionally, the systemclock PLL 13 uses the system reference signal to generate a system clocksignal that controls timing of the IC 40. Although not illustrated inFIG. 1 for clarity of the figure, the system clock signal can be used tocontrol timing of a wide variety of circuits of the IC 40, including,but not limited to, the reference demodulators 2 a-2 d, the TDCs 4 a-4d, the DPLLs 6 a-6 b, the reference monitors 18, the reference switchingcircuit 19, and/or the auxiliary TDCs 22.

The modulation and phase offset controller 14 provides a wide variety offunctionality. For example, the modulation and phase offset controller14 can control a division rate and/or phase delay of the output dividers9 a-9 e, thereby controlling frequency and phase of the output clocksignals. The modulation and phase offset controller 14 of FIG. 1 alsocontrols clock bursting for supporting gapped-clock applications.Furthermore, the modulation and phase offset controller 14 controlsmodulation of the location of output clock edges to selectively insert asubcarrier into a higher frequency carrier clock signal. Implementingthe modulation and phase offset controller 14 in this manner aids ingenerating a reference clock signal having a carrier frequency and anembedded subcarrier frequency.

The temperature sensor 15 operates to generate a temperature indicationsignal indicating a temperature of the IC 40, for instance, atemperature condition near or local to the system clock PLL 13. In theillustrated embodiment, the temperature indication signal is provided tothe system clock compensation circuit 16, which operates to generatecompensation signals for compensating one or more circuit blocks of theIC 40 for error of the system clock signal arising from temperaturevariation.

With continuing reference to FIG. 1, the reference monitors 18 operateto detect whether or not one or more of the reference clock signalsreceived on the input reference pins (REFA, REFAA, REFB, REFBB) arereliable. For example, the IC 40 can be programmed with tolerance data(for instance, via the serial port) associated with a tolerated amountof reference clock jitter permitted for a particular application.Additionally, the reference monitors 18 can process digital time stampsfrom the TDCs 4 a-4 d to determine whether or not a particular one ofthe input reference clock signals are reliably operating within theallotted tolerance.

The reference switching circuit 19 aids in controlling which inputreference clock signals are provided as inputs to the DPLLs 6 a-6 b. Forexample, in a variety of applications, multiple reference clock signalsare provided for redundancy and/or other reasons. Additionally, when aparticular reference clock signal is unavailable or becomes unreliable,the reference clock signal can be switched. In certain implementations,a DPLL is temporarily operated open loop in a holdover mode duringreference switching, thereby stabilizing the output clock signalsgenerated by the IC 40 and preventing sudden output frequency changes.

The auxiliary NCOs 21 and the auxiliary TDCs 22 operate to provideon-chip NCOs and TDCs for a wide variety of functions, thereby expandingthe flexibility and/or range of applications that the IC 40 can be usedin.

The status and control pins interface 23 operates as an interface fortransmitting and receiving signals over the multi-functional pins (MPINS).

The serial port and memory controller 24 is coupled to a serial port orinterface, such as a serial peripheral interface (SPI) orinter-integrated circuit (I²C) interface. The serial port and memorycontroller 24 can be used for a wide variety of functions, including,but not limited to, for receiving data from the user associated withprogramming or configuring the IC 40 in a desired manner.

The clock synchronization and frequency translation IC 40 can be used tocontrol clocking and timing in a wide variety of applications. In oneexample, the IC 40 provides jitter cleanup and synchronization in GPS,PTP (IEEE-1588), and/or SyncE applications. In a second example, theclock synchronization and frequency translation IC 40 is included in abase station (for instance, a femtocell or picocell) to control clockingfor baseband and radio. In a third example, the clock synchronizationand frequency translation IC 40 controls mapping/demapping for atransport network, such as an optical transport network (OTN), whileproviding jitter cleaning. In a fourth example, the clocksynchronization and frequency translation IC 40 provides holdover,jitter cleanup, and phase transient control for Stratum 2, 3e, and 3applications. In a fifth example, the clock synchronization andfrequency translation IC 40 provides support for data conversionclocking, such as analog-to-digital (A/D) and/or digital-to-analog (D/A)conversion, for instance, for JESD204B support. In a sixth example, theclock synchronization and frequency translation IC 40 provides timingfor wired infrastructure support, such as cable infrastructure and/orcarrier Ethernet.

The clock synchronization and frequency translation IC 40 illustratesone embodiment of a semiconductor chip that can be implemented inaccordance with one or more features discussed herein. However, theteachings herein are applicable to other implementations of electronicsystems.

FIG. 2A is a schematic diagram of one implementation of a DPLL 50 for aclock synchronization and frequency translation IC, such as the clocksynchronization and frequency translation IC 40 of FIG. 1. The DPLL 50includes a digital phase detector 51, a digital loop filter 52, an NCO53, and a feedback divider 54.

The DPLL 50 of FIG. 2A illustrates one example of a DPLL suitable foruse as the DPLLs 6 a, 6 b of FIG. 1. However, the DPLLs 6 a, 6 b of FIG.1 can be implemented in other ways.

The digital phase detector 51 compares a digital reference signal 55 toa digital feedback signal 56 to generate a numeric phase error signal.In certain implementations, the digital phase detector 51 includes aTDC-based phase detector. In one example, a first TDC generates digitaltime stamps representing time instances at which the digital referencesignal 55 transitions and a second TDC generates digital time stampsrepresenting time instances at which the digital feedback signal 56transitions, and the digital phase detector 51 processes the time stampsto generate the numeric phase error signal. In another example, thedigital phase detector 51 generates a digital error signal based oncomparing the digital reference signal 55 to the digital feedback signal56, and a common TDC is used to generate time stamps representingtransitions of the digital error signal.

The digital loop filter 52 provides digital filtering to the numericphase error signal based on one or more numeric coefficients to generatea numeric frequency tuning word (FTW). As shown in FIG. 2A, the numericFTW serves as an input to the NCO 53. In certain implementations, thedigital loop filter 52 has a programmable loop bandwidth to enhanceflexibility.

With continuing reference to FIG. 2A, the NCO 53 receives a system clocksignal, such as from the system clock PLL 13 of FIG. 1. The NCO 53generates a DPLL output clock signal 57 based on the system clock signaland a value of the numeric FTW. As the value of the numeric FTW changes,a frequency of the system clock signal changes correspondingly.

The feedback divider 54 generates the digital feedback signal 56 basedon dividing the DPLL output clock signal 57. In certain implementations,the feedback divider 54 operates with a programmable divisor value toenhance flexibility.

In steady state, the DPLL 50 locks the phase of the digital referencesignal 55 to the phase of the digital feedback signal 56.

Although FIG. 2A illustrates one implementation of a DPLL, DPLLs can beimplemented in a wide variety of ways.

FIG. 2B is a schematic diagram of one implementation of an APLL 60 for aclock synchronization and frequency translation IC, such as the clocksynchronization and frequency translation IC 40 of FIG. 1. The APLL 60includes a phase detector 61, a loop filter 62, a voltage controlledoscillator (VCO) 63, and a feedback divider 64.

The APLL 60 of FIG. 2B illustrates one example of an APLL suitable foruse as the APLLs 7 a, 7 b of FIG. 1. However, the APLLs 7 a, 7 b of FIG.1 can be implemented in other ways.

The phase detector 61 operates to generate an analog phase error signalbased on comparing a reference clock signal 65 to a feedback clocksignal 66. In certain implementations, the reference clock signal 65corresponds to the DPLL output clock signal 57 of FIG. 2A. The phasedetector 61 can be implemented in a wide variety of ways. In oneexample, the phase detector 61 includes a phase-frequencydetector/charge pump (PFD/CP) that controls a flow of current into orout of the loop filter 62 based on comparing the reference clock signal65 to the feedback clock signal 66.

The loop filter 62 generates a control voltage used to control anoscillation frequency of the VCO 63. The loop filter 62 has a loopbandwidth, which can be fixed or controllable, based on implementation.The VCO 63 generates an APLL output clock signal 67, which is divided bythe feedback divider 64 to generate the feedback clock signal 66. Incertain implementations, the feedback divider 64 has a programmabledivisor.

When in lock, the APLL 60 operates to phase-lock the feedback clocksignal 66 to the reference clock signal 65. Additionally, a divisionrate of the divider 64 can be selected to control frequency translationof the APLL output clock signal 67 relative to the reference clocksignal 65.

Although FIG. 2B illustrates one implementation of an APLL, APLLs can beimplemented in a wide variety of ways.

FIG. 2C is a schematic diagram of one implementation of a system clockPLL 70 for a clock synchronization and frequency translation IC, such asthe clock synchronization and frequency translation IC 40 of FIG. 1. Thesystem clock PLL 70 includes a system reference control circuit 71, aPFD/CP/loop filter 72, a VCO 73, a feedback divider 74, a lock detector75, and a VCO calibration circuit 76.

In the illustrated embodiment, the system reference control circuit 71includes an input multiplexer 41, a maintaining amplifier 42, a firstinput amplifier 43, a second input amplifier 44, a multiplexer 45, afrequency doubling circuit 46, a divider 47 (divider by 1, 2, 4 or 8, inone implementation), and an output multiplexer 48.

The system clock PLL 70 of FIG. 2C illustrates one example of a systemclock PLL suitable for use as the system clock PLL 13 of FIG. 1.However, the system clock PLL 13 of FIG. 1 can be implemented in otherways.

As shown in FIG. 2C, the PFD/CP/loop filter 72, VCO 73, and feedbackdivider 74 (divide by 4 to 255, in one implementation) operates as aninteger-N frequency synthesizer that generates the system clock signalbased on a system reference signal 78. In one example, the VCO operateswith a frequency range of 2250 megahertz (MHz) to 2415 MHz. However,other frequency operating ranges are possible.

The system reference pins (XOA, XOB) serve in providing a desired systemreference signal to the system clock PLL 70. In one example, a user canconnect a crystal resonator to the XOA and/or XOB pins, and themaintaining amplifier 42 provides energy sufficient to maintainoscillations of the crystal resonator. In another example, a user canconnect a single-ended and/or differential clock source (for instance, atemperature compensated crystal oscillator (TCXO) or oven controlledcrystal oscillator (OXCO)) to the system reference pins, and the systemreference control circuit 71 generates the system reference signal 78based on a reference clock signal received from the clock source.

As shown in FIG. 2C, the system reference control circuit 71 can providefrequency translation of the system reference signal received on thesystem reference pins (XOA, XOB). For instance, in the illustratedembodiment, the system reference signal can be optionally doubled ordivided. Implementing the system reference control circuit 71 in thismanner provides flexibility in a range of system reference frequencies,while satisfying an operational input frequency range of the PFD/CP/loopfilter 72 and/or frequency tuning range of the VCO 73.

The system clock PLL 70 of FIG. 2C includes the lock detector 75, whichindicates when a feedback clock signal 79 from the feedback divider 74is locked to the system reference signal 78.

With continuing reference to FIG. 2C, the system clock PLL 70 alsoincludes the VCO calibration circuit 76, which operates to configure theVCO 73 for particular system clock parameters via a calibrationsequence.

Although FIG. 2C illustrates one implementation of a system clock PLL,system clock PLLs can be implemented in a wide variety of ways.

FIG. 3 is a schematic diagram of another implementation of a DPLL 80 fora clock synchronization and frequency translation IC, such as the clocksynchronization and frequency translation IC 40 of FIG. 1. The DPLL 80includes a digital phase detector 51, a digital loop filter 52, an NCO53, and a feedback divider 54, which can be as described earlier withrespect to FIG. 2A. The DPLL 80 further includes a reference TDC 81, afeedback TDC 82, a holdover switch 83, a FTW processor 84, a loopcontroller 85, and a lock detector 86.

The DPLL 80 illustrates another example of a DPLL suitable for use asthe DPLLs 6 a, 6 b of FIG. 1, with the reference TDC 81 corresponding toone or more of the TDCs 4 a-4 d and/or auxiliary TDCs 22 of FIG. 1.However, the DPLLs 6 a, 6 b of FIG. 1 can be implemented in other ways.

The reference TDC 81 generates reference digital time stamps 91representing time instances of transitions of an input reference signal89. Additionally, the feedback TDC 82 generates feedback digital timestamps 92 representing time instances of transitions of a feedbacksignal 90 from the feedback divider 54. The digital phase detector 51compares the reference digital time stamps 91 to the feedback digitaltime stamps 92 to generate a numeric phase error signal representing aphase error between the input reference signal 89 and the feedbacksignal 90.

The illustrated DPLL 80 also includes the lock detector 86, whichgenerates a lock detect signal indicating whether or not the inputreference signal 89 and the feedback signal 90 are locked to oneanother. The digital loop filter 52 processes the numeric phase errorsignal to generate a numeric FTW that is provided to the FTW processor84 via the holdover switch 83.

As shown in FIG. 3, the loop controller 85 controls the holdover switch83, thereby controlling whether or not the DPLL 80 operates closed loopor open loop. In the illustrated embodiment, the loop controller 85controls an operating mode of the DPLL 80 to a selected operating modechosen from multiple different operating modes including at least aphase-locking mode (closed loop) and a holdover mode (open loop).Furthermore, the loop controller 85 aids in providing seamlesstransition when transition from one operating mode to another operatingmode.

For example, reference switchover occurs when the input reference signal91 is changed from one input reference signal to another. For instance,the input reference signal 91 can be provided to the DPLL 80 by aselection circuit, such as a multiplexer, and the selected referencesignal can be changed for a variety of reasons. When handling areference switchover, the loop controller 85 can briefly enter aholdover mode, in which the holdover switch 83 is opened, new DPLLparameters are updated, and the FTW processor 84 operates with aholdover FTW from the loop controller 85. Thereafter, the holdoverswitch 83 is closed and the DPLL 80 operates closed-loop with the newinput reference signal.

The loop controller 85 can also operate the DPLL 80 indefinitely in theholdover mode when all of the input references are invalid and/or when auser manually sets or forces the holdover mode, for instance, byprogramming via a serial port. In the holdover mode, the outputfrequency of the DPLL 80 remains substantially fixed, althoughinstability of the system clock signal can lead to output frequencyvariation.

After recovery from the holdover mode, the loop controller 80 restoresthe DPLL to closed-loop operation and locks to the input referencesignal, including recovery of loop parameters based on the profilesettings for the new input reference signal after a switchover.

The FTW processor 84 processes a received FTW from the digital loopfilter 52 or loop controller 85 to generate a FTW for the NCO 53. TheFTW processor 84 can provide a number of functions, such as programmabledelay, statistical processing (for instance, windowed averaging), and/ortuning word history.

Although FIG. 3 illustrates one implementation of a DPLL, DPLLs can beimplemented in a wide variety of ways.

FIG. 4 is a schematic diagram of one implementation of a NCO 100 for aclock synchronization and frequency translation IC, such as the clocksynchronization and frequency translation IC 40 of FIG. 1. The NCO 100generates an NCO output clock signal based on a system clock signal andan input FTW. The NCO 100 includes an FTW conversion circuit 95, a sigmadelta modulator (SDM) 96, and a tuning word filter 97.

The NCO 100 illustrates one example of an NCO suitable for use in theDPLLs 6 a, 6 b of FIG. 1, for serving as the auxiliary NCOs 21 of FIG.1, and/or for serving as the NCO 53 of FIGS. 2A and 3. However, NCOs canbe implemented in other ways.

The FTW conversion circuit 95 converts the input FTW (k bits) into aninteger tuning portion (m bits) and a fractional tuning portion (nbits). In one example, k is 48 bits, m is 4 bits, and n is 40 bits.However, other implementations are possible.

As shown in FIG. 4, the integer tuning portion is provided to an integerinput to the SDM 96, and the fractional tuning portion is filtered bythe tuning word filter 97 and thereafter provided to a fractional inputto the SDM 96. The SDM 96 provides sigma-delta modulation to generatethe NCO output clock signal (F_(NCO)) based on the system clock signaland values of the integer and fractional tuning portions.

Although FIG. 4 illustrates one implementation of an NCO, NCOs can beimplemented in a wide variety of ways.

FIG. 5 is a schematic diagram of one implementation of frequencytranslation loops 150 for a clock synchronization and frequencytranslation IC 100. Although the frequency translation loops 150 areillustrated in the context of the IC 100 of FIG. 5, the clocksynchronization and frequency translation IC 40 of FIG. 1 can also beadapted to include the frequency translation loops 150.

The IC 100 includes a first input reference buffer 101 a, a second inputreference buffer 101 b, a first reference divider 103 a, a secondreference divider 103 b, a first reference TDC 104 a, a second referenceTDC 104 b, an auxiliary TDC 104 c, another DPLL feedback TDC 104 d, aDPLL 106, an APLL 107, an output clock distribution circuit 110, asystem clock PLL 113, a first multiplexer 121, a second multiplexer 122,and a third multiplexer 123. The clock synchronization and frequencytranslation IC 100 further includes input reference pins (REFX, REFY),system reference pins (XOA, XOB), and a clock output pin (OUTX).

Although one example of circuitry and pins is shown for a clocksynchronization and frequency translation chip, other implementationsand circuitry and/or pins can be used.

The DPLL 106 includes a time stamp processor 131, a digital loop filter132, a tuning word processor 133, an NCO 134, a feedback divider 135,and a feedback TDC 136. Additionally, the APLL 107 includes a PFD/loopfilter 141, a VCO 142, and a feedback divider 153. In the embodimentshown in FIG. 5, the feedback divider 135 of the DPLL 106 receives anoutput clock signal from downstream via the third multiplexer 123,rather than directly from the DPLL's NCO.

In the illustrated embodiment, the DPLL 106 operates in part bycomparing digital time stamps from a TDC selected by the firstmultiplexer 121 to digital time stamps from a TDC selected by the secondmultiplexer 122. Additionally, a DPLL output clock signal from the DPLL106 serves as an input reference signal to the APLL 107. The APLL outputclock signal from the APLL 107 in turn is provided to the outputdistribution circuit 110, which provides an output clock signal to theclock output pin OUTX.

As shown in FIG. 5, the DPLL 106 operates with a selectable feedbackpath to the time stamp processor 131. For example, the DPLL 106 includesa first feedback path 145 from the output of the feedback divider 143 ofthe APLL 107, a second feedback path 146 from the output distributioncircuit 110, and a third feedback path 147 corresponding to an off-chippath from the clock output pin OUTX to the REFY input reference pin.

The multiplexers 122-123 can be used to select a desired feedback path,thereby helping to achieved an output-to-input phase alignment desirablefor a particular application.

System Clock Compensation

Autonomous oscillators, such as those used to provide local frequencyreferences, suffer from multiple impairments to their average and/orinstantaneous accuracy. For example, the system reference signalsreceived on the system reference pins (XOA, XOB) of FIGS. 1, 2C, and 5can suffer from such frequency stability and accuracy errors.

The average frequency accuracy (or simply, accuracy) may be described asthe center or nominal frequency, which may be offset from the idealtarget frequency value. Short-term frequency accuracy can be viewed as adeviation from the average and thus as a relative frequency stability.In certain implementations, accuracy is a manufacturing constant thatvaries per device and stability is environmentally correlated.Environmental factors include, but are not limited to, temperature,mechanical acceleration (vibration), mechanical stress, and time(aging).

In certain implementations herein, an IC includes a system clockcompensation circuit that generates one or more compensation signals fordesensitizing various circuit blocks of the IC from actual variation infrequency in a system reference signal based on a time-varying estimateof the frequency error. By implementing the IC in this manner, the ICoperates with less sensitivity to frequency variation in the systemreference signal. For example, frequency stability and accuracy errorsof a system clock signal generated from the system reference signal canbe accommodated by desensitizing circuit blocks clocked by the systemclock signal from such errors.

Thus, with a suitable model, accuracy and/or stability errors of anoscillator can be estimated. When a secondary frequency reference isavailable, the relative accuracy and/or stability can be alternativelyor additionally measured. When the errors of the secondary reference aresmall, relative measurements can be used to estimate the primaryreference's errors. The estimation techniques may be combined to improvetheir overall quality.

For example, let Ê_(f)(t) designate the LO's estimated fractionalfrequency error versus time; that is, for an ideal frequency, f₀, theactual frequency is f(t)≈f₀×(1+Ê_(f)(t)). Note that one or more otherenvironmental factors that affect the oscillator may also be expressedas functions of time, so this is a general form and not restrictive ofthe underlying model or method of determining this value. Furthermore,Ê_(f)(t) is one expression of the LO's error, it may be represented inother forms and other units, such as an offset frequency or fractionalperiod error.

One method of generating Ê_(f)(t) is to characterize the LO's frequencyas a function of an environmental parameter, such as temperature, T(which is a time variant value, though the function of time notation isomitted for brevity). The characterization data can be fit to apolynomial function of a desired order, and Ê_(f)(t) expressed asÊ_(f)(T)=A₀+A₁×T+A₂×T²+ . . . or as another suitable function. Thefunction used for modeling can be extended to account for otherparameters, such as aging, supply voltage, etc. Although an example witha polynomial is described, other functions are possible, such asfunctions associated with other numbers of variables and/orcomputational complexities.

Another method of determining the LO's frequency error is to measure theerror relative to another clock signal which is designated as moreaccurate and/or more stable (this clock signal will be referred to as astable clock or stable reference). Since the qualities which contributeto a good LO are not always found in conjunction with accuracy andstability, a clock that possesses these attributes, but would not be agood candidate as an LO, can be used to determine Ê_(f)(t).

The measurement can be achieved by a wide variety of techniques. In oneexample, the stable clock is applied as a reference to a digital phaselock loop (DPLL) operating at a rate derived (for instance, directly)from the LO. In various embodiments of DPLLs, the numeric control wordgenerated by the loop is a time variant representation of the frequencyratio of the LO clock and the stable clock. Thus, the numerical controlword can be processed to calculate a fractional frequency error and acorresponding compensation signal for compensating a particular circuitblock for the fractional frequency error. One advantage of using a DPLLfor detecting clock error is that the DPLL's loop applies a low-passfilter to the stable clock signal. This is a significant advantage whenthe stable clock exhibits substantial phase jitter, as the filter willreduce or minimize the noise in Ê_(f)(t).

One example of a component or circuit block that exhibits a directdependence on oscillator frequency error is an NCO. Real time clocks(RTCs) are an application of NCOs. NCOs are generally constructed aroundone of two core elements, either an accumulator—commonly a phaseaccumulator such as used in a direct digital synthesizer (DDS), or asigma-delta modulator (SDM), as used in a significant number offractional-N PLLs. Given Ê_(f)(t), each of these core components can becompensated.

For instance, in an accumulator based NCO, the average output frequencyof the component can be given by:f_(nco)(t)=f(t)×control_word÷control_modulus, where control_word andcontrol_modulus can be determined at design time and/or provided duringoperation. When it is desirable to generate an ideal frequency,f_(nco_ideal), which is not specifically ratio-metric to f(t), an idealcontrol word can be calculated,control_word_ideal=control_modulus×f_(nco_ideal)÷f₀. Combining this withprior equations, and assuming that control_modulus is substantiallyconstant, a compensated control word can be computing which yieldsf_(nco)(t)≈f_(nco_ideal): control_word=control_word_ideal÷(1+Ê_(f)(t)).

An SDM based NCO provides an average output frequency that can be givenby: f_(nco)(t)=f(t)×control_modulus÷control_word. This too can be solvedfor an equation giving a compensated control word, for instance,control_word=control_word_ideal×(1+EÊ_(f)(t)).

Another example of components with oscillator rate dependence arediscrete time (sampled) filters, which have coefficients described asfunctions of the sample frequency. For instance, a single pole IIRfilter has a coefficient, a, which determines the −3 dB point of itsfrequency response, f_(c)(t). This relationship is given byf_(c)(t)≈f(t)×α/(2π×(1−α)), assuming f_(c)(t)<<f(t). For a fixed−3 dBpoint, f_(c_ideal), α=₂π×f_(c_ideal)/(f₀×(1+Ê_(f)(t))).

FIG. 6 is a schematic diagram of one embodiment of an electronic system210 with system clock compensation. The electronic system 210 includes asystem clock generation circuit 201, a system clock compensation circuit202, and clocked circuit blocks 203 a, 203 b, . . . 203 n.

The electronic system 210 can represent a wide variety of electronicsystems. In one embodiment, the electronic system 210 represents aportion of a semiconductor chip used for clock synchronization and/orfrequency translation, such as the clock synchronization and frequencytranslation IC 40 of FIG. 1.

The system clock generation circuit 201 generates a system clock signalbased on timing of a system reference signal. The system clock signal isused for controlling timing of a wide variety of circuitry, includingclocked circuit blocks 203 a, 203 b, . . . 203 n, in this example.

The clocked circuit blocks 203 a, 203 b, . . . 203 n can represent awide variety of components, including, but not limited to, filters(including, but not limited to, sampled filters), reference monitors,TDCs, DPLLs (including, but not limited to, all digital phase-lockedloops or ADPLLs), and/or NCOs. Although an embodiment with three clockedcircuit blocks is shown, more or fewer clocked circuit blocks can becompensated as indicated by the ellipses.

The system reference signal may be inaccurate and/or unstable, which canlead to a time dependent error of the system clock signal. For example,a system clock can have an error arising from an operating condition,such as temperature and/or another environmental factor. Absentcompensation, the error in the system clock signal can lead to error inoutput signals generated by the clocked circuit blocks 203 a, 203 b, . .. 203 n.

The illustrated electronic system 210 includes the system clockcompensation circuit 202, which generates compensation signals COMP1,COMP2, . . . COMPn for compensating the clocked circuit blocks 203 a,203 b, . . . 203 n from error the system clock signal. The system clockcompensation circuit 202 can be implemented in a wide variety of ways,including implementations that provide open-loop compensation,closed-loop compensation, or a combination thereof.

In various embodiments, the compensation signals COMP1, COMP2, . . .COMPn are digital signals, such that the system clock compensationcircuit 202 digitally compensates one or more clocked circuit blocks forsystem clock error.

The system clock compensation scheme of FIG. 6 avoids a need tosynthesize a clean clock signal from the system clock signal. Rather,system clock stability compensation can be provided by using the systemclock signal as a timing source for circuit blocks, and compensating thecircuit blocks for frequency errors of the system clock signal. Thus,frequency stability and accuracy errors of a system clock signal can beaccommodated by digitally compensating the circuit blocks.

FIG. 7 is a schematic diagram of another embodiment of an electronicsystem 230 with system clock compensation. The electronic system 230includes a system clock PLL 211, a system clock compensation circuit212, a TDC 213, a filter 214, a DPLL 215, an NCO 216, and a referencemonitor 217.

The electronic system 230 can represent a wide variety of electronicsystems, for example, a portion of a clock synchronization and/orfrequency translation IC.

The system clock PLL 211 generates a system clock signal based on timingof a system reference signal. The system clock PLL 211 can beimplemented in a wide variety of ways, including, but not limited to,using the system clock PLL 70 of FIG. 2C.

The system clock signal is used for controlling timing of a wide varietyof circuitry, including the TDC 213, the filter 214, the DPLL 215, theNCO 216, and the reference monitor 217, in this example. Although aspecific example of clocked circuit blocks is shown, system clockcompensation can be used to desensitize a wide variety of clockedcircuit blocks from system clock error.

In the illustrated embodiment, the system clock compensation circuit 212receives one or more signals indicating one or more operating conditionsof the electronic system. Examples of operating conditions includesupply voltage and/or environmental factors, such as temperature,mechanical acceleration (vibration), mechanical stress, and/or time(aging).

The system clock compensation circuit 212 further includes an errormodel 221, which generates a modeled system clock error based on the oneor more signals indicating operating conditions. The system clockcompensation circuit 212 further includes a system clock errorcalculation circuit 222, which processes the modeled system clock errorto generate compensation signals for various circuit blocks. In thisexample, the circuit blocks include the TDC 213, the filter 214, theDPLL 215, the NCO 216, and the reference monitor 217. However, othercircuit blocks and/or different combinations of circuit blocks can becompensated for error of a system clock signal. The system clock errorcalculation circuit 222 generates compensation signals of valuessuitable for compensating various clocked circuit blocks for anestimated system clock error.

The error model 221 can be implemented in a wide variety of ways. In oneembodiment, the error model 221 corresponds to a polynomial model.However other types of models can be used.

Coefficients of the error model 221 can be obtained in a wide variety ofways. In one example, a user can program an IC (for instance, via theserial port of the IC 40 of FIG. 1) with model data suitable formodeling a particular frequency reference. In another example, the modelcoefficients are adaptively learned. In yet another example, modelparameters are programmed into the device during factory test and/ormanufacture.

Thus, the system clock compensation circuit 212 operates using the errormodel 221 to reduce sensitivity of various components to system clockerror arising from a change in temperature, supply voltage, and/or otheroperating condition.

A system clock compensation circuit that provides compensation based onan error model can also be referred to herein as providing open-loopcompensation for system clock error.

FIG. 8 is a schematic diagram of another embodiment of an electronicsystem 240 with system clock compensation. The electronic system 240includes a system clock PLL 211, a TDC 213, a filter 214, a DPLL 215, anNCO 216, a reference monitor 217, a clock difference calculation circuit231, and a system clock compensation circuit 232.

The electronic system 240 can represent a wide variety of electronicsystems, for example, a portion of a clock synchronization and/orfrequency translation IC.

In the illustrated embodiment, the electronic system 240 receives notonly a system reference signal for the system clock PLL 211, but also astable reference signal for the clock difference calculation circuit231.

When a secondary reference signal is available (for instance, the stablereference signal shown in FIG. 8), the relative frequency accuracyand/or stability of the system clock signal can be measured relative tothe secondary reference signal. When the frequency errors of thesecondary reference signal are small, the relative measurements can beused to accurately estimate the system clock signal's error. Theestimation technique may be combined with open-loop compensation toimprove the overall accuracy of compensation.

As shown in FIG. 8, the clock difference calculation circuit 231compares the stable reference signal to the system clock signal togenerate a measured clock error signal. The clock difference calculationcircuit 231 can be implemented in a wide variety of ways, such as byusing a PLL. The measured clock error signal is processed by a systemclock error calculation circuit 233 of the system clock compensationcircuit 232 to generate compensation signals for one or more circuitblocks that are clocked by the system clock signal.

A system clock compensation circuit that provides compensation based ona measured clock error can also be referred to herein as providingclosed-loop compensation for system clock error.

FIG. 9 is a schematic diagram of another embodiment of an electronicsystem 260 with open loop system clock compensation. The electronicsystem 260 includes an internal temperature sensor 251, a multiplexer252, multipliers 253 a-253 i, adders 254 a-254 e, a filter 255, and amemory 256.

The electronic system 260 can represent a wide variety of electronicsystems, for example, a portion of a clock synchronization and/orfrequency translation IC.

The electronic system 260 provides closed-loop system clock compensationby generating a digital compensation signal for compensating for amodeled system clock error arising from temperature. As shown in FIG. 9,the memory 256 stores model coefficients 257 for the error model, suchas polynomial model coefficients C1, C2, C3, C4, and C5. The memory 256can be implemented using any type of elements that store data,including, but not limited to, volatile memory cells, non-volatilememory cells, registers, fuses, and/or any other suitable type of datastorage elements.

As shown in FIG. 9, the multiplexer 252 is controlled by selectionsignal SEL. The multiplexer 252 selects between an external temperaturevalue and a temperature value from the internal temperature sensor 251as an input to the error model. The temperatures are P-bit digitalsignals, in this example. In one implementation, P is sixteen. However,other implementations are possible.

The error model uses the temperature input signal and the coefficients257 to generate a model estimate, which is further processed by thefilter 255 to generate the digital compensation signal. The filter 255aids in mitigating noise injection, and in certain implementations has auser-controllable filtering characteristic.

The digital compensation signal is based on the value of the model at aparticular temperature, and thus can vary with time as the temperaturechanges. The digital compensation signal is used by one or more circuitblocks to provide desensitization to stability and accuracy errors ofthe system clock signal. Although illustrated as generating onecompensation signal, multiple compensation signals can be generated.

Although an example with temperature is shown, the teachings herein areapplicable to compensation of system clock errors arising from otheroperating conditions, for instance, supply voltage and/or environmentalfactors. In another embodiment, an error model is configured to receivea vibration signal indicating a vibration condition. For example, anaccelerometer can be used to detect vibration and provide the errormodel with a vibration signal indicating the amount of vibrationpresent. In yet another embodiment, an error model is configured toreceive a supply voltage signal indicating a supply voltage condition.In yet another embodiment, multiple indicators of operating conditionsare provided to an error model.

FIG. 10 is a schematic diagram of an IC 275 with closed loop systemclock compensation. The IC 275 includes an input reference buffer 261,an input reference divider 262, a reference TDC 263, a feedback TDC 264,a digital PFD and loop filter 265, a compensation calculator 266, an NCO267, a feedback divider 268, and a system clock error calculationcircuit 269.

The IC 275 measures an error of the system clock signal based oncomparing the system clock signal to a stable reference source using aDPLL. Thus, the DPLL of FIG. 10 serves to measure a clock error of thesystem clock signal relative to the stable reference signal, which isreceived from the REFX pin, in this example.

As shown in FIG. 10, FTWs from the DPLL are processed by the systemclock error calculation circuit 269 to generate one or more compensationsignals for circuit blocks of the IC 275.

The DPLL of FIG. 10 includes the compensation calculator 266, whichprocesses the FTWs from the digital PFD and loop filer 265 to controlthe input FTW to the NCO 267.

In the illustrated embodiment, the compensation calculator 266 is alsoconfigured to receive a secondary compensation signal, which can beprovided to further enhance accuracy of the clock error measurement. Inone example, the secondary compensation signal is provided from an errormodel, thereby combining open-loop and closed-loop system clockcompensation. In another example, the secondary compensation signal isprovided from another PLL loop that compares the system clock signal toanother stable reference signal.

FIG. 11 is a schematic diagram of a system clock compensation circuit280 according to another embodiment. The system clock compensationcircuit 280 includes an error model 221, a clock difference calculationcircuit 231, and a combiner 276.

The error model 221 generates an open loop estimate of an error of thesystem clock signal based on values of one or more operating conditions(for instance, temperature, supply voltage, and/or other operatingconditions). Additionally, the clock difference calculation circuit 231generates a closed loop estimate of the error of the system clock signalbased on comparing the system clock signal to the stable referencesignal.

In the illustrated embodiment, the combiner 276 generates one or morecompensation signals based on combining the estimates from the errormodel 221 and the clock difference calculation circuit 231. In certainimplementations, a system clock compensation circuit includes a slewrate limiter 277 to limit a rate at which one or more compensationsignals can change. For instance, the slew rate limiter 277 can be usedto limit a maximum change in frequency and/or phase of an output clocksignal generated by an IC.

The system clock compensation circuit 280 illustrates one example of asystem clock compensation circuit that operates using a combination ofopen-loop compensation and closed-loop compensation.

FIG. 12 illustrates a schematic diagram of a TDC 281 according to oneembodiment. The TDC 281 includes an accumulator 282 that receives acompensation signal COMP that controls an accumulation rate of the TDC281.

The TDC 281 provides time-to-digital conversion of an input signal INbased on timing of a system clock signal. The TDC 281 generates digitaltime stamps representing time instances at which transitions (forinstance, rising and/or falling edges) of the input signal occur.

The accumulator 282 is used in generating the digital time stampsrepresenting the time instances of the input signal transitions.Additionally, the compensation signal COMP is used to vary a rate of theaccumulator 282 to maintain the average accumulator slope substantiallyconstant, thereby providing compensation based on instantaneousfrequency error of the system clock signal. By implementing the TDC 281in this manner, the digital time stamps can be substantially independentof temperature or other operating conditions.

By enhancing the performance of the TDC 281, a number of benefits can beachieved. In one example, the time stamps from the TDC 281 are used by aDPLL to control phase locking. By compensating the TDC 281 using thecompensation signal, superior DPLL performance can be achieved. Inanother example, the time stamps from the TDC 281 can be processed by areference monitor to determine whether or not a reference signal (forinstance, an external clock signal provided by a user) is within aspecified frequency accuracy. By providing a TDC that is desensitized tosystem clock error, the reference monitor can achieve referencemonitoring with higher accuracy, lower latency, and/or at a resolutionfiner than a ppm variation of the system clock signal.

FIG. 13 is a schematic diagram of a DPLL 285 according to anotherembodiment. The DPLL 285 of FIG. 13 is similar to the DPLL 80 of FIG. 3,except that the DPLL 285 includes a FTW processor 284 that receives acompensation signal COMP from a system clock signal compensationcircuit.

The compensation signal provides digital adjustment to the FTW. In oneexample, the compensation signal can be added to the input tuning wordreceived by the FTW processor 284. Additionally, the compensation signalhas a value selected to desensitize the FTW processor 284 to stabilityand accuracy errors of the system clock signal.

When a system clock signal is compensated for temperature and/or otheroperating conditions, a lower loop bandwidth for the DPLL 285 can beused, which results in better loop filtering of low frequency phasenoise and/or relaxed constraints on a quality or precision of an inputreference signal to the DPLL 285.

Thus, such compensation techniques allow a DPLL loop to providefiltering of loop noise and/or input reference noise without beingconstrained or limited by a need to filter for errors in the systemclock signal.

FIG. 14 is a schematic diagram of an NCO 290 according to anotherembodiment. The NCO 290 of FIG. 14 is similar to the NCO 100 of FIG. 4,except that the NCO 290 includes an FTW processor 289 that receives acompensation signal from a system clock signal compensation circuit.

The NCO 290 generates an output clock signal based on the FTW and thesystem clock signal. Additionally, the compensation signal provides anadjustment or modification to the control word such that the NCO iscompensated for error in the system clock signal arising from changes intemperature, supply voltage, and/or operating conditions.

Reducing Variation in Signal Propagation Delay

Signal propagation speeds through a signal path including active devicesand/or transmission media can vary for a number of reasons. For example,a delay in signal propagation can vary with temperature, frequency,and/or other operating conditions.

In certain implementations herein, signal timing within an electronicsystem is adjusted based on modeled and/or measured propagation delay.By enhancing signal timing in this manner, a reduction in signal timingvariation at one or more desired nodes or points within a system can beachieved.

While described in the context of controlling timing of a clock signal,the teachings herein are applicable to reducing variation in signalpropagation delay of other kinds of signals. In one example, digitalsignals can be retimed to the timing of a clock signal, and thus can beindirectly controlled via that clock signal's timing.

Certain PLLs are configured such that there is a well-controlled phasedifference between an input reference signal to the PLL and an outputclock signal generated by the PLL. For example, zero-delay PLLs refer toPLLs in which a phase difference between the output clock signal and theinput reference signal is about zero degrees. Zero-delay PLLs canoperate without output frequency scaling, such that a phase differenceof zero degrees is present for each cycle. Zero-delay PLLs can alsooperate with output frequency scaling, such that the zero phase pointsof the signals match on regular subsets of clock cycles.

A zero-delay PLL can minimize a phase difference between the outputclock signal and input reference clock signal by matching the effectivedelay in the PLL's feedback path to equivalent portions of the referenceand output signal paths.

The physical location where the phases nominally align can be adjustedby including all or part of the output signal path, or a replicathereof, within the feedback path. Alternately or additionally, acontrollable timing element can be included in the PLL and used to shifta point of phase alignment.

When the reference signal path and output signal path vary in delay, thealignment will vary to a degree that is based on a quality of delaymatching between these paths and the feedback path. For example, pathmatching can rely on the paths experiencing the same variation inducingstimulus and reacting equivalently. Generally, an (asymmetric) timingelement in the PLL, such as a phase shifter, which is not equallyrepresented in the path delays, will result in a non-equivalent reactionin the paths.

By including a timing control element (for instance, a controllabledelay element) capable of live adjustment (for example, adjustment thatchanges over time during operation) into a signal path, new methods ofachieving zero delay at a point in the system and of reducing delayvariation at that point can be achieved. For example, the timing controlelement can be included in a PLL or any other suitable periodic signalpath. For a periodic signal, any delay of multiple unit cycles isindistinguishable in certain applications, so the minimum insertiondelay of an element may be effectively removed. Non-periodic signals arealso subject to the same techniques with regard to reducing delayvariation.

Signal delay to any point in the system may be modeled, empirically orotherwise, with any number of variables. Evaluating this delay model andapplying the negative of the result to the timing control element cansubstantially control the net delay to zero or another desired phasevalue. When model variables change, the model evaluation and delayadjustment process can be repeated to maintain the desired delay.Furthermore, any delay sensitivities of the timing control element canbe included in the delay model.

A wide variety of delay models can be used in accordance with theteachings herein. For example, delay models may span a wide range ofcomplexity. In certain implementations, a zero variable model is used tocorrect for a nominal offset.

The delay models can compensate for variation in delay arising from avariety of sources. In one example, the delay model receives an inputtemperature signal indicating a temperature condition.

When compensating for temperature, a single locality in the system canbe used to approximate the average response over a larger physicalregion and/or multiple temperature measurements can be used to accountfor gradient effects.

Additionally or alternatively to temperature, other variables can bemodeled for impact on delay variation, such as device supply voltage(s)and/or signal format (i.e. CMOS, LVDS, etc.). Furthermore, evenvariables associated with timing measurements can be included. Forexample, a round trip delay time, such as measured by a time domainreflectometer or other suitable detector, can be used as a modelvariable. In contrast to zero delay PLLs whereby part of the feedbackpath encompasses a portion of the output path or a replica thereof, themodel variable can be measured independent of the feedback path.

Delay adjustment can be provided by a wide variety of timing controlelements capable of live adjustment. For example, certain DPLLs includedelay adjustable circuitry. The origination point of a signal, such asan oscillator, can also be delay adjustable in certain implementations.Furthermore, certain NCOs, including but not limited to direct digitalsynthesizers (DDS), have adjustable delays.

In certain implementations, adjustment to a DPLL is controlled in adifferent manner depending on whether or not the timing loop of the DPLLis active. For example, an active loop's phase offset is subject to thedynamic response of the DPLL. In certain implementations, a DPLLincludes an NCO, and a coordinated adjustment of the DPLL phase and NCOphase is provided for an active DPLL.

FIG. 15 is a schematic diagram of one embodiment of an electronic system410 with delay compensation. The electronic system 410 includes an IC401 including an output pin 405, a timing circuit 406, and a delaycompensation circuit 408. The timing circuit 406 generates an outputsignal (OUT) based on timing of a reference signal REF. Additionally,the output signal is provided to a destination node 402 via the outputpin 405 and a path 403 with variable delay.

In certain implementations, the IC 410 is a clock synchronization andfrequency translation IC, such as the clock synchronization andfrequency translation IC 40 of FIG. 1. In such implementations, thetiming circuit 406 can include a DPLL, and the output signal provided tothe output pin 405 can correspond to an output clock signal.

The illustrated IC 410 includes the delay compensation circuit 408,which generates a compensation signal COMP that compensates the timingcircuit 406 for a variation in the delay of the path 403. In certainimplementations, the delay compensation circuit 408 further compensatesfor on-chip variation.

The delay compensation circuit 408 can compensate for variation in delayarising from temperature, supply voltage, and/or other operatingconditions. In certain implementations, the compensation signal COMP isdigital, such that the delay compensation circuit 408 provides digitaladjustment for delay variation. Additionally, the value for digitaladjustment is changed over time to compensate for variation in delay.

By implementing the IC 401 in this manner, a difference in phase betweenthe output signal at the destination node 402 and input reference signalREF can be controlled to a desired value.

FIG. 16 is a schematic diagram of another embodiment of an electronicsystem 430 with delay compensation. The electronic system 430 includesan IC 411 including a DPLL 415, a delay compensation circuit 416, aninput reference pin 417, and an output clock pin 418. The DPLL 415receiving an input reference signal REF from the input reference pin417, and generates an output clock signal that is provided to the outputclock signal pin 418. Although not illustrated in FIG. 16, variousdividers, multiplexers, buffers, and/or other circuitry can be presentbetween the DPLL 415 and the IC's pins. As shown in FIG. 16, the outputclock signal is provided to a destination node 412 via an externalsignal path 413, which can have a delay that varies with one or moreoperating conditions. Although an example of providing delaycompensation to an external signal path is shown, delay compensation canbe provided to a signal path including internal elements, externalelements, or a combination thereof. Thus, delay compensation isapplicable to internal signal paths, external signal paths, and signalpaths including both an internal portion and an external portion.

In certain implementations, the IC 411 is a clock synchronization andfrequency translation IC, such as the clock synchronization andfrequency translation IC 40 of FIG. 1.

A zero-delay PLL generates an output clock signal with substantially thesame phase as an input reference clock signal from which the outputclock signal is synthesized. However, a zero-delay PLL may not besuitable for certain applications. For example, a zero-delay PLL can beincluded on an IC, and a clock output signal from the IC can be providedover long trace and/or through other components (for instance,additional chips). Additionally, it can be desirable to match the phaseat a certain point off chip to the phase of the input reference clocksignal.

Such off-chip routes and components can have delays that vary withtemperature, supply voltage, and/or other operating conditions. Althougha zero-delay PLL may drive the inputs of its phase detector to aboutzero degrees, the phase at one or more points off chip may not have adesired phase relationship with respect to the input reference signal.

The illustrated IC 411 includes the delay compensation circuit 416,which generates a compensation signal that compensates the DPLL 415 fora variation in the delay of the signal path 413. In certainimplementations, the delay compensation circuit 416 further compensatesfor on-chip variation.

In the illustrated embodiment, the delay compensation circuit 416includes a delay model 423 and a delay error calculation circuit 424.

As shown in FIG. 16, the delay model 423 receives one or more signalsindicating one or more operating conditions of the electronic system.Examples of operating conditions include supply voltage, signal format,and/or environmental conditions, such as temperature, moisture, and/orhumidity.

The delay model 423 generates a modeled delay error based on the one ormore signals indicating operating conditions. Additionally, the delayerror calculation circuit 424 processes the modeled delay error togenerate a compensation signal for compensating the delay of the DPLL415. The delay compensation circuit 416 can provide delay adjustmentusing the compensation signal COMP in a wide variety of ways, includingboth within a loop of a DPLL and/or outside of such a loop.

The delay model 423 can be implemented in a wide variety of ways. In oneembodiment, the delay model 423 corresponds to a polynomial model.However, other types of models can be used.

Coefficients of the delay model 423 can be obtained in a wide variety ofways. In one example, a user can program an IC (for instance, via theserial port of the IC 40 of FIG. 1) with model data suitable formodeling a particular external signal path, including transmission mediaand/or active components.

Thus, the delay compensation circuit 415 of FIG. 16 uses the delay model423 to estimate the change in delay of the signal path 413 arising froma change in temperature, supply voltage, and/or other operatingcondition. Additionally, a corresponding digital adjustment is providedsuch that the delay is compensated for, and the desired clock signalphase is maintained at the desired off-chip destination node 412.

A delay compensation circuit that provides compensation based on andelay model can also be referred to herein as providing open-loopcompensation for delay variation.

FIG. 17 is a schematic diagram of another embodiment of an electronicsystem 440 with delay compensation. The electronic system 440 includesan IC 431 including an input reference pin 417, an output pin 418, areturn path pin 419, a DPLL 415, a delay compensation circuit 432, and adelay difference detector 433. As shown in FIG. 17, the output clocksignal is provided to a destination node 412 via an external signal path413, which can have a delay that varies with one or more operatingconditions. Additionally, a return path 414 is provided from at or nearthe destination node 412 to the return path pin 419.

In certain implementations, the IC 431 is a clock synchronization andfrequency translation IC, such as the clock synchronization andfrequency translation IC 40 of FIG. 1.

In the illustrated embodiment, the delay difference detector 433measures or detects a delay difference between the output clock signalfrom the DPLL 415 and the return clock signal received on the returnpath pin 419. The delay difference detector 433 can be implemented in avariety of ways, such as using a TDC. The detected difference signal isprocessed by a delay error calculation circuit 434 of the delaycompensation circuit 432.

The delay error calculation circuit 434 determines a delay error at thedestination point 412 corresponding to a delay variation of the signalpath 413 arising from temperature and/or other operating conditions. Thedelay error calculation circuit 434 takes into account a total pathlength from the output clock pin 418 to the return path pin 419. Forexample, in implementations in which a full round trip is made, theround trip total path length can be about twice that of the length ofthe signal path 413, and thus the delay error calculation circuit 434can appropriately divide the detected delay error by a factor of about2.

Accordingly, in certain implementations, variation in delay iscompensated for using measured delay variation. In one example, aroundtrip trace can be routed from an output pin of a chip to an inputpin of a chip to detect about twice of the off-chip delay. Additionally,about half such a delay can be digitally compensated for. In anotherexample, a trace of shorter length is used to provide an estimation ofonly a portion or section of the off-chip delay.

The calculated delay error is used to generate a compensation signalCOMP for the DPLL 415, such that the phase as the destination node 412is controlled to a desired value. The delay compensation circuit 432 canprovide delay adjustment using the compensation signal COMP in a widevariety of ways, including both within a loop of the DPLL 415 or outsideof the DPLL's loop.

A delay compensation circuit that provides compensation based on ameasured delay difference can also be referred to herein as providingclosed-loop compensation for delay variation.

FIG. 18 is a schematic diagram of another embodiment of a clocksynchronization and frequency translation IC 450. The clocksynchronization and frequency translation IC 450 of FIG. 18 is similarto the clock synchronization and frequency translation IC 40 of FIG. 1,except that the clock synchronization and frequency translation IC 450further includes a delay compensation circuit 448. In this example, thedelay compensation circuit 448 receives a temperature indication signalfrom the temperature sensor 15. In certain implementations, the delaycompensation circuit 448 includes a delay model, which can be programmedby a user via the serial port.

FIG. 19 is a schematic diagram of another embodiment of an IC 460 withdelay compensation. The IC 460 includes a memory 256, a delaycompensation circuit 456, and a DPLL 458.

The delay compensation circuit 456 includes an internal temperaturesensor 451, a multiplexer 452, multipliers 453 a-453 i, adders 454 a-454d, and a filter 455. As shown in FIG. 19, the memory 256 stores modelcoefficients 457 for the delay model, such as polynomial modelcoefficients C1, C2, C3, C4, and C5. Additionally, the multiplexer 452selects between an external temperature value and a temperature valuefrom the internal temperature sensor 451 as an input to the error model.The error model uses the temperature input signal and the coefficients457 to generate a model estimate, which is further processed by thefilter 455 to generate the digital compensation signal COMP. The filter455 aids in mitigating noise injection, and in certain implementationshas a user-controllable filtering characteristic (for instance,controllable by data provided over a serial port). The digitalcompensation signal COMP is based on the value of the delay model at aparticular temperature, and thus can vary with time as the temperaturechanges. Although illustrated as generating one compensation signal,multiple compensation signals can be generated.

Although an example with temperature is shown, the teachings herein areapplicable to compensation of delay errors arising from other operatingconditions, for instance, supply voltage, signal format, and/orenvironmental factors.

The DPLL 458 if FIG. 19 is similar to the DPLL 80 of FIG. 3, except thatthe DPLL 458 further includes an adder 459. In the illustratedembodiment, the adder 459 combines the feedback time stamps from thefeedback TDC 82 with the digital compensation signal COMP and auser-controllable phase offset signal (PHASE OFFSET).

Although one example of providing phase adjustment to a DPLL is shown, aphase of a DPLL can be adjusted in a wide variety of ways, includingadjustment within the DPLL's loop, adjustment outside of the DPLL'sloop, or a combination thereof.

In one example, an output of the DPLL's phase detector is digitallyadjusted based on a digital compensation signal. In another example, areference input to a phase detector (for instance, values of digitaltime stamps) is digitally adjusted to provide phase adjustment. In yetanother example, an explicit digitally-controllable delay element (forinstance, a digitally-controlled delay line or DDL) is used forproviding such adjustment.

FIG. 20 is a schematic diagram of another embodiment of an electronicsystem 490 with delay compensation. The electronic system 490 includesan IC 471 including a reference pin 480, a first clock output pin 481, asecond clock output pin 482, a first digitally-controllable delayelement 483 (a DDL, in this example), a second digitally-controllabledelay element 484, a DPLL 485, and a delay compensation circuit 486.

In the illustrated embodiment, the DPLL 483 generates an output clocksignal that is provided to the first output clock pin 481 via the firstdigitally-controllable delay element 483 and to the second output clockpin 482 via the second digitally-controllable delay element 484.Additionally, the output clock signal travels from the first output pin481 to a first destination node 473 via a first signal path 475, and toa second destination node 474 via a second signal path 476.

The first and second signal paths 475, 476 can have different nominaldelays and/or delay variations with operating conditions. In theillustrated embodiment, the delay compensation circuit 486 providesseparate digital compensation signals COMP1, COMP2 to thedigitally-controllable delay elements 483, 484, respectively.

Using digitally controllable delay elements (for instance, DDLs) outsidethe loop of a DPLL or other timing circuit can provide independent orseparate control in implementations including multiple output pinsrouted to different destinations.

In certain implementations, control over one or more digitallycontrollable delay elements is combined with digital adjustment to aDPLL via another compensation signal. For example, certain digitallycontrollable delay elements may have settings that are relatively coarseand/or that exhibit a setting-dependent jitter. Thus, certainimplementations combine phase adjustment via digitally controllabledelay elements outside of a DPLL's loop with a phase adjustment to aDPLL's loop.

Reference Monitors with Dynamically Controlled Latency

The average period of a clock signal can be estimated by measuring atime interval between two points of the clock signal, and dividing bythe number of clock cycles present between the points.

However, an accurate time-base is needed to accurately measure aninterval of time. For example, measured time intervals can beratio-metrically related to the time-base, and thus any deviation inaverage accuracy will be related to the time-base. Instantaneousdeviations in accuracy, as characterized by measurement jitter, canarise from the time-base and/or the clock measurement device thatgenerates measurements based on the time-base.

The measured clock signal is also subject to phase jitter, whichcombines with the measurement jitter to provide an uncertainty in themeasured time interval. As the duration of measurement increases (forinstance, increasing a number of cycles of the clock signal over whichthe measurement is taken), the uncertainty becomes small relative to thetime interval. Thus to achieve a smaller uncertainty, a longermeasurement duration can be used.

A value of the uncertainty can be assumed, and a measurement latency canbe chosen to be sufficiently long to account for the assumeduncertainty. However, using an assumed uncertainty value has significantdrawbacks. For example, when the assumed uncertainty value is too large,the measurement duration is needlessly long. However, when the assumeduncertainty is smaller than the actual uncertainty, the measured timeinterval has a precision that violates the required tolerance, and thusmay be inaccurate.

In certain implementations herein, a clock measurement circuit operatesto generate digital measurements of a reference clock signal based ontiming of a system clock signal. Additionally, a reference monitorprocesses the digital measurements from the clock measurement circuit tostatistically estimate the uncertainty in the digital measurements.Additionally, the reference monitor uses the estimate of the uncertaintyto control a latency of the reference monitor in detecting whether ornot the reference clock signal is within a tolerance specified by one ormore tolerance parameters.

In certain applications, an estimate of the uncertainty can be detectedin less time than it takes to satisfy the measurement toleranceindicated by the tolerance parameters. Thus, the reference monitor candetect whether or not the reference clock signal is within the tolerancewith lower latency and higher speed.

Reducing a latency of the reference monitor can provide a number ofadvantages. For example, quickly determining whether or not thereference clock signal is within the tolerance provides a system withadditional time to react to the change in the clock signal status, forinstance, in response to a change of the reference clock signal fromwithin the tolerance to outside of the tolerance.

Furthermore, for larger shifts in frequency, the reference monitor candetect when the reference clock signal is out of tolerance more quickly.For example, when the error band around the frequency measurement nolonger intersects with the tolerance window around the expectedfrequency, the reference monitor can determine that the reference clocksignal has failed the monitoring comparison. Although the error banddecreases in size over longer intervals, a relatively large error bandmay still cause the comparison to fail. For example, because themeasured value can be further from the ideal to accommodate the largerband, larger frequency offsets can be detected earlier.

Since the measurement is of the average frequency over the interval,longer intervals are less sensitive to shifts in frequency in the shortterm. Once the desired precision is met, the measurement can beconcluded, and a new measurement can begin. In certain implementations,the reference monitor performs frequency measurements over multipleoverlapping intervals simultaneously, thereby improving reaction time.

Allowing for near minimal observation intervals, quickly detectinglarger shifts in frequency, and/or maintaining sensitivity to shifts infrequency aid in reducing frequency detection latency.

FIG. 21 is a schematic diagram of one embodiment of a referencemonitoring system 610. The reference monitoring system 610 includes aclock measurement circuit 601 and a reference monitor 602.

As shown in FIG. 21, the clock measurement circuit 601 receives areference clock signal (REF CLOCK). The reference clock signal can bereceived in a variety of ways, such as from a pin of an IC. Although thereference clock signal is illustrated as being directly provided to aninput of the clock measurement circuit 601, in certain implementationsthe reference clock signal is processed prior to measurement. Forexample, the reference clock signal can be buffered, divided, inverted,and/or processed in other ways.

The clock measurement circuit 601 generates digital measurements(DIGITAL MEASUREMENTS) of the reference clock signal based on timing ofa system clock signal (SYSTEM CLOCK). Thus, the system clock signalserves as a time base for generating the digital measurements.

The reference monitor 602 processes the digital measurements from theclock measurement circuit 601 to determine whether or not the referenceclock signal is within a desired tolerance specified by one or moretolerance parameters (TOLERANCE). In certain implementations, thetolerance parameters indicate at least one of a tolerated error infrequency accuracy or a tolerated error in frequency stability.

As shown in FIG. 21, the reference monitor 602 generates a monitoroutput signal (MONITOR OUT) indicating whether or not the referenceclock signal is within the tolerance. Additional signals may beoutputted, for instance, signals that quantify the particular toleranceviolated, in which direction, and by what magnitude.

The reference monitor 602 includes a statistical processing circuit 603,which processes the digital measurements from the clock measurementcircuit 601 to statistically estimate the uncertainty in the digitalmeasurements. Additionally, the statistical processing circuit 603 usesthe estimate of the uncertainty to control a latency 604 of thereference monitor 602 in detecting whether or not the reference clocksignal is within the tolerance indicated by the one or more toleranceparameters.

The statistical processing circuit 603 can use a wide variety ofstatistical processing, including, but not limited to, calculation ofvariance and/or mean of the digital measurements over a time window.Persons of ordinary skill in the art will appreciate that a statisticalprocessing circuit can calculate variance directly or indirectly bycalculating standard deviation and/or another statistical indicator ofvariation.

Uncertainty in measurements can arise from a number of sources, such asjitter of the system clock signal and/or jitter of the clock measurementcircuit 601. By dynamically controlling the monitor's latency based onthe estimate of the uncertainty, the delay of the reference monitor 602can be dynamically adjusted as needed to obtain a desired measurementconfidence.

For example, when the uncertainty of measurement is estimated to berelatively low, the statistical processing circuit 603 shortens thelatency 604 of monitor 602, thereby generating the monitor output signalrelatively rapidly while maintaining a desired confidence. However, whenthe uncertainty of the measurement is estimated to be relatively high,the statistical processing circuit 603 extends the latency 602 such thatthe reference monitor 602 determines whether or not the reference clocksignal is within the tolerance with a desired confidence interval.

In contrast, a reference monitor that operates with an assumed or fixeduncertainty value can exhibit poorer performance. For example, when theassumed uncertainty value is too large, the measurement duration isneedlessly long. However, when the assumed uncertainty value is smallerthan the actual uncertainty, the measured value has a precision thatviolates the required tolerance, and thus measurement device cangenerate unreliable measurements.

In one embodiment, the reference monitor 602 has an initial estimate ofthe uncertainty, which can be obtained in a variety of ways, such as viauser programming and/or implemented in the reference monitor 602 as partof design and/or during manufacture. For a given selection of toleranceparameters, the reference monitor 602 can have a nominal latencycorresponding to the initial estimate of the uncertainty. Furthermore,the statistical processing circuit 603 lengthens or shortens the latency604 of the frequency measurement relative to the nominal latency basedon the estimated uncertainty.

FIG. 22 is a schematic diagram of another embodiment of a referencemonitoring system 620. The reference monitoring system 620 of FIG. 22 issimilar to the reference monitoring system 610 of FIG. 21, except thatthe reference monitoring system 620 illustrates an implementation inwhich the clock measurement circuit is implemented as a TDC 611 thatprovides digital time stamps (TIME STAMPS) to a DPLL 613.

The TDC 611 provides time-to-digital conversion of the reference clocksignal based on timing of the system clock signal. The reference monitor602 processes the digital time stamps from the TDC 611 to determinewhether or not the reference clock signal is reliable. Additionally, theDPLL 613 processes the digital time stamps to control phase locking.

There can be a trade-off between the accuracy of a reference monitor anda latency of the reference monitor. For example, the reference monitorcan observe a reference clock signal over a relatively long window oftime to determine the reliability of the reference clock signal with arelatively high confidence. In contrast, the reference monitor canobserve the reference clock signal over a relatively short time window,but such an estimate may result in the reference monitor incorrectlydetermining whether or not the reference clock signal is within adesired tolerance.

In the illustrated embodiment, the reference monitor 602 observes thedigital time stamps to determine whether or not the reference clocksignal is within a tolerance. For example, a user can specify anexpected periodicity and tolerance for error, for instance, 1microsecond and 1 ppm, respectively. Additionally, the reference monitor602 dynamically adjusts the length of the measurement window or latency604 based on statistically processing the output of the TDC 611. Thus,the reference monitor 602 can be used to observe the period of output ofthe TDC 611, and to develop statistics of the period over multiplecycles of the reference clock signal. Additionally, a statistical modelcan be used to determine the length of observation.

The length of observation can be reduced when the statistics indicatethat a measurement error arising from jitter is less than expected.Additionally, the length of observation can be increased when thestatistics indicate that the measurement error arising from jitter isgreater than expected. Thus, not only can the latency of a referencemonitor be reduced, but the latency can be extended when an upper limitor bound on jitter error was incorrected chosen. Accordingly, teachingsherein can be used to provide robust reference monitoring, rather thanbeing inoperable by an inaccurate upper bound to jitter error selectedduring design.

FIG. 23 is a schematic diagram of another embodiment of a referencemonitoring system 670. The reference monitoring system 670 includes areference clock buffer 671, a reference clock divider 672, a TDC 673, areference monitor 674, a programmable validation timer 675, and outputlogic circuitry 676.

In the illustrated embodiment, the reference buffer 671 buffers thereference clock signal REFA. The buffered clock signal is provided tothe reference clock divider 672, which operates to selectively dividethe reference clock signal REFA by a divisor R_(A). The TDC 673processes the reference clock signal from the divider 672 to generatedigital time stamps for the reference monitor 674. In one embodiment,the digital time stamps are also provided to a digital cross pointmultiplexer (for example, the digital cross point multiplexer 5 of FIG.1). In another embodiment, the digital time stamps are obtained from thedigital cross point multiplexer, rather than directly from the TDC 673.

The reference monitor 674 processes the digital time stamps to determinewhether or not the reference clock signal REFA is within a desiredtolerance as indicated by various tolerance parameters. In this example,the tolerance parameters include a nominal period T_(REF), an offsetlimit ΔTOL relative to the nominal period, a hysteresis threshold ΔHYS,and a jitter limit J_(TOL). However, other implementations of toleranceparameters are possible. In certain implementations, the toleranceparameters are provided via user input, for instance, via programming aninterface, such a serial port.

In the illustrated embodiment, the reference monitor 674 generatesoutput signals indicating whether or not the reference clock signal REFAis within the desired tolerance. Furthermore, the output logic circuitry676 provides further logical processing to generate various statussignals of the reference clock signal REFA.

In this example, the status signals include an excess jitter signalJ_(EXCESS) indicating whether or not the clock reference signal REFA isoutside the jitter limit J_(TOL), a loss of reference signal LOSindicating whether or not the clock reference signal REFA has been lost,a fast signal FAST indicating whether or not the clock reference signalREFA is outside of the offset limit ΔTOL for being too fast, and a slowsignal SLOW indicating whether or not the clock reference signal REFA isoutside of the offset limit ΔTOL for being too slow. The status signalsfurther includes a fault signal OOT, which indicates when the referenceclock signal REFA fails any of the tolerance parameters and thus isoutside of the tolerance.

The status of the reference clock signal REFA (as indicated by one ormore of the status signals) can be used to control a variety offunctions of an IC, such as automatic reference switching. For instance,when the reference monitoring system 670 is implemented in the IC 40 ofFIG. 1, the status of one or more reference clock signals received onthe input reference pins (REFA, REFAA, REFB, REFBB) can be provided tothe reference switching circuit 19 to control reference switching. Thestatus of the reference clock signals can also be provided to one ormore pins, for instance, via the serial port pins (SERIAL PORT) and/ormultifunction pins (M PINS).

In the illustrated embodiment, the reference monitor 674 serves tomonitor the reference clock signal REFA for both frequency accuracy andfrequency stability. For example, the offset limit ΔTOL can indicate amaximum amount the period may deviate from the nominal period T_(REF),and thus can be used to monitor for frequency accuracy. Furthermore, thejitter limit J_(TOL) indicates a maximum amount of jitter (for instance,root mean square jitter), and thus can used to monitor for frequencystability.

Implementing the offset limit ΔTOL as a proportional value rather thanan absolute offset can provide a number of advantages. For example, thereference monitor 682 can detect whether or not the reference clocksignal REFA is in tolerance without needed to know the division value ofthe reference divider 672. Rather, the reference monitor 674 cancontinually observe the difference between successive time stamps andcompare the statistics of those observations relative to the offsetlimit ΔTOL to determine the reliability of the reference signal, forinstance, whether the reference is fast, slow, absent, and/or exhibitsexcessive jitter.

With continuing reference to FIG. 23, the reference monitor 674 furtherreceives a ΔHYS signal, which is used by the reference monitor 674 afterthe reference clock signal REFA has faulted (is outside of thetolerance). For example, the ΔHYS signal can be used to determine amaximum amount the period may deviate from T_(REF) for a faultedreference clock signal before indicating that the reference clock signalREFA is no longer faulted (within the tolerance).

Jitter introduces uncertainty in the time measurement between successiveedges (for instance, successive rising edges) of the reference clocksignal REFA. Furthermore, the internal time base (for example, thesystem clock signal) that the TDC 673 uses to make its measurements alsointroduces jitter uncertainty.

Both jitter sources compromise a monitor's ability to determine when thereference is truly in or out of tolerance. That is, jitter dilutes theconfidence of the monitor's ability to make an accurate decision.Furthermore, a reference monitor may have no a priori knowledge of themagnitude or spectral distribution of the jitter present on a particularreference clock signal. Furthermore, jitter characteristics can varyover time.

The illustrated reference monitor 674 includes the statisticalprocessing circuit 681, which processes the numeric time stamps from theTDC 673 to measure the reference period, including calculatingstatistics (mean and variance, in this embodiment) of the referenceclock signal REFA as it observes period samples.

The statistical processing circuit 681 uses the calculated statistics toestimate the actual uncertainty of measurement arising from jitter. Forexample, by comparing the calculated variance with the offset limitΔTOL, the statistical processing circuit 681 determines how many periodsamples are needed to arrive at a period estimate with sufficientconfidence to decide whether or not the reference clock signal REFA isin or out of tolerance. Thus, the statistical processing circuit 681dynamically varies the latency 682 of the monitor 674 based on thecalculated statistics.

Implementing the reference monitor 674 with the statistical processingcircuit 681 enables the reference monitor 674 to perform a highconfidence period estimate of the reference clock signal REFA withreduced or near optimal minimum observation time. In certainimplementations, the reference monitor 674 is implemented as a statemachine.

In certain implementations, the statistical processing circuit 681generates estimates of uncertainty over multiple time windows that arepartially overlapping, thereby providing multiple simultaneous frequencymeasurements to further improve reaction time.

In one embodiment, rather than measuring the actual period of thereference clock signal REFA, the statistical processing circuit 681estimates a deviation of the period relative to a scaled value of thenominal period T_(REF) to a sufficiently high degree of confidence.Accordingly, certain statistical algorithms can have a confidencerequirement integrated into its design.

Under certain conditions, a decision time of a marginally out oftolerance reference signal can vary with a ratio of the square root ofthe measured jitter variance to the offset limit ΔTOL. Thus, the smallerthe variance of the measured jitter, the less averaging and the shorterthe decision making time.

The statistical processing circuit 681 controls the latency 682 of themonitor 674 based on processing the time stamps to estimate theuncertainty of measurement. Thus, the latency or decision delay of themonitor 674 is not deterministic, but rather based on the value of theoffset limit ΔTOL and the actual jitter present.

Although jitter plays a role in the decision time of the referencemonitor 674 under normal operation, it has relatively little effect onthe decision time when the reference period is much greater than thescaled TREF value (that is, TREF multiplied by the divisor of thedivider 672) or when the reference signal disappears completely. Forexample, the reference monitor 674 operates on time stamps from the TDC673, and thus a loss of the clock reference signal REFA stalls thereference monitor's period estimation process.

In the illustrated embodiment, the reference monitor 674 uses its localtime base and the scaled TREF value to predict when the next edge willarrive. When the edge is relatively late (for instance, 15% beyond theprediction of the next reference input edge), the monitor 674 activatesthe loss of reference signal LOS. Implementing the monitor 674 in thismanner aids in detecting when the reference clock signal REFA is nolonger present.

In the illustrated embodiment, the reference monitor 674 generates thefault signal OOT indicating whether or not the reference clock signalREFA is within tolerance. After the reference clock signal REFA faults,the reference monitor 674 monitors the reference signal for an intolerance condition. When the reference clock signal REFA returns totolerance, the fault signal OOT is controlled to indicate a non-faultcondition.

For certain applications, it is desirable for the reference clock signalREFA to be in a non-faulted condition for a period of time before thereference clock signal REFA is considered to be usable or valid. Toaccommodate such applications, the reference monitor 674 includes theprogrammable validation timer 675.

For example, the programmable validation timer 675 can start in responseto the reference monitor 674 transitioning the fault signal OOP from afaulted state to a non-faulted state. Thereafter, the timer 675 countsdown, and activates the valid signal VALID to indicate that thereference is available for use in response to expiration of the timer.In this example, the programmable validation timer 675 receives thetimer validation signal T_(VALID) indicated a countdown period of thetimer 675. The value of T_(VALID) can be provided by the user in wholeor in part; augmentation of this value can be included to accommodatevariation in the monitor's detection latency. In the illustratedembodiment, a user also can force a start condition via a fault signalFAULT and a bypass signal BYPASS.

In the illustrated embodiment, the programmable validation timer 675stops counting down whenever the reference status becomes faulted (asindicated by the fault signal OOT). A subsequent start event results inthe timer 675 reinitiating a count down from the programmed value of thetimer validation signal programmable. Thus, in the example, thereference clock signal REFA does not attain valid status unless thereference clock signal REFA remains in tolerance for the full durationof the validation timer.

The illustrated timer 675 further receives the timeout signal TIMEOUT,which can be used to force the timer 675 to end its count. In thismanner, if a faulted reference has returned to a non-faulted state andis awaiting validation, the user can override the timer, if desired,thereby bringing the reference clock signal to valid status.

Precision Timing Distribution and Recovery

Timing can be distributed within an electronic system by way oftransition edges of a reference signal, such as a level shifts ofvoltage or current, or pulses of light.

However, since the edges can occur at any point in the continuum oftime, such timing information is inherently analog. Although a widevariety of information in an electronic system is represented digitallyand is able to benefit from fast and/or dense digital data transfertechniques, timing remains stubbornly analog.

Analog-to-digital converters (ADCs) provide a method to representvoltages or currents as ratios of well-defined quantities, the Volt andAmp. Not only are the magnitude of these values relatively easy toapproximate locally, they have easy to represent zero values. Time andthe second are more difficult to meter and there is no universallyaccepted, locally available, zero.

Two converters can be said to agree if their local meters, of whatevermeasurement unit, do not differ by more than a specified number ofminimum fractional units, or least significant bits (LSBs). The greaterthe dynamic range, or number of significant bits in the conversionresult, the more difficult it is for two converters to agree. Forexample, 8-bit converters can be made to agree with relative ease,16-bit converters may require careful matching and trim to agree, beyondthat, more exotic techniques may be needed.

In certain implementations herein, timing of transitions of a referencesignal is digitized using a time-to-digital converter (TDC). Forexample, the TDC can generate digital time stamps representing timeinstances at which transitions (for instance, rising and/or fallingedges) of the reference signal occur.

Certain TDCs may have a dynamic range with a resolution of 1 picosecondand full-scale of 1 microsecond to 1 millisecond or more, for instance,20-30+bits. Even in the presence of a convenient local time meter,obtaining agreement between such TDCs can be challenging.

Crystal oscillators or other autonomous frequency generators can be usedas a local time meter. However, crystal oscillators can be ofinsufficient accuracy for many applications. For example, typicalreferences are accurate to tens of parts per million (ppm) or worse,which can correspond to less than 17 bits of resolution. Furthermore,instantaneous accuracy varies substantially with environmental factors,such as temperature.

To overcome such limitations, an electronic system can operate withoutlocal reference dependency and share a common reference between alldevices exchanging TDC samples. Regardless of the absolute accuracy andstability of the common reference, as long as all devices share the sameimpairments, they can agree on TDC values, although specificapplications may have bounds on absolute accuracy.

In certain implementations herein, source devices provide destinationdevices with TDC time stamps. Additionally, the source devices areimplemented with a format conversion circuit for interpreting digitaltime stamps consistently throughout the system.

Certain applications operate with timing achieved using both frequencyinformation (periods of signals) and phase information. Sharing bothfrequency and phase information requires a common sense of zero time.Unlike other physical quantities, zero time may be defined cyclically,like January 1st, midnight, or the start of every hour. By providingboth frequency and phase information, timing information can beunambiguously interpreted, for instance, when represented relative tothe prior or next zero.

When timing information is transmitted between a sender and receiverthere is a delay between the composition of the message and thecomprehension of that message. In order for the message to be understoodunambiguously, the zero value to which the message is relative should beknown. If the cyclic duration between zeros is much longer than thelongest transmission delay (uncertainty), the correct zero can beproperly determined.

In certain implementations, a fixed transmission delay can becharacterized and accounted for. For example, consider a message sentwith the time X units prior to zero. If this message is received shortlybefore a zero, it can be assumed that the zero referred to by themessage is the next zero. However, if the message is received shortlyafter a zero, it can be determined that the message referred to thatzero which just passed. Any other inferences for zero would imply atransmission delay greater than, or approaching, the cyclic period ofzeros.

A specific timing event on a shared timing meter reference can bedesignated as zero, therefore allowing it to be a complete timingreference. When the periodicity of the reference is much lower than themessage transmission delays, then every event can be designated as zero.However, there are many practical considerations which make thisconfiguration unrealistic. Unfortunately, any reference rate faster thanthe message propagation delay cannot use (unidirectional) messagingalone to designate which subset of events represent zero. Zero eventscan be indicated via a parallel analog timing signal or by embeddingindicator data within the timing reference signal itself.

With a shared timing meter reference, intervals between timing eventscan be shared within a system. With a complete timing reference,individual timing events can be shared. Which timing information isshared and thus the relative complexity of the timing solution can bebased on application.

Techniques for distributing and utilizing a shared timing reference,both with and without zero synchronization, are described in greaterdetail herein. In certain implementations, electronic systems that arecapable of conveying both frequency and phase timing information areprovided. Furthermore, provided herein are algorithms and applicationsthat can be built on top of such systems.

FIG. 24 is a schematic diagram of an electronic system 810 according toanother embodiment. The electronic system 810 includes a first sourcedevice 801 a, a second source device 801 b, a first destination device802 a, a second destination device 802 b, a data hub 803, a commontime-base 804, and local oscillators (LOs) 805 a-805 d.

The first source device 801 a receives a first signal (signal A), acommon clock signal from the common time base 804, and a first localclock signal from the LO 805 a, which are used to transmit dataindicating timing of the signal A over the data hub 803. Additionally,the second source device 801 b receives a second signal (signal B), thecommon clock signal from the common time base 804, and a second localclock signal from the LO 805 b, which are used to transmit dataindicating timing of the signal B over the data hub 803. Although anexample with two source devices is shown, more or fewer source devicescan be included.

As shown in FIG. 24, the first destination device 802 a receives datafrom the data hub 803, the common clock signal from the common time base804, and a third local clock signal from the LO 805 c, which are used torecover signal A and/or signal B with proper timing. Additionally, thesecond destination device 802 b receives data from the data hub 803, thecommon clock signal from the common time base 804, and a fourth localclock signal from the LO 805 d, which are used to recover signal Aand/or signal B with proper timing.

The electronic system 810 can be used to digitally distribute a periodictiming signal, either physical or logical, from one point in the system810 to another point in the system 810 where it is regenerated, eitherlogically or physically. The regenerated or recovered signal possessesthe precise average frequency of the original and may also replicate itsphase to within a certain accuracy.

The system 810 allows for the sharing of digital timing informationacross distributed devices. An application of the system 810 is toreplicate qualities of a periodic signal (frequency and optionallyphase) from one point to one or more additional points by means ofdigital data transfer rather than analog signal transfer.

For example, utilizing analog signaling, network equipment chassis,timing cards, and line cards are designed with knowledge of the chassissize (maximum number of line cards). Backplane connectors vary based onthe maximum number of ingress/egress ports per card. With analogsignaling, these systems can be made less sensitive to the total sizeand port count of the chassis, but at the expense of functionality.

In contrast, physical data connections (lanes) are much more flexibleand efficient than physical clock signals due to packetizedmultiplexing, carrier detect multiple access (CDMA), error detection andcorrection, and/or scalable throughput rate. Migrating timinginformation from clock signals to data lanes has clear advantages inscalability.

Sending the signal information requires continually capturing timinginformation about the source signal and encapsulating that data in amessage transmitted to the receiver. Timing for a physical signal can becaptured, for instance, via a TDC, while logical (non-physical) signalevent timing may be determined in a variety of ways. The timinginformation may be for all events, i.e. all rising and falling edges ofthe signal waveform, a regular subset of events, i.e. only the risingedges or only every Nth rising edge, or an irregular subset of events.

When timing information of an irregular subset of events is provided,additional information can be sent along with the message whichidentifies the subset, which may be explicit or implicit (inferredcontextually). In one example, the sender may indicate how manyintermediate events were skipped, or, if the nominal period is knownwith sufficient accuracy, the receiver may be able to infer the numberof skipped events. In another example, each event can be serialized withan incrementing value such that the difference between identifiersindicates the number of events, thereby providing limited loss ofmessages to occur between the sender and receiver without damaging theintegrity of the sequence. Other information about the events may alsobe included, such as identifying a specialized subset.

Regeneration of periodic analog timing signals can be performed in avariety of ways, such as by delay or phase lock loops (DLLs or PLLs), ora combination thereof. For digital timing signals, the digitalequivalent of these loops can be used (DDLLs or DPLLs). The output ofeither digital loop can be logical or physical. Both loops can usedigital phase detectors (DPDs) to determine the error vector between thesource signal and the regenerated signal.

In certain implementations, the source signal is the digital timing datareceived from the remote source and the regenerated signal isrepresented by a time stamp produced by the local logical or physicalclock output. The error vector is used to control (often indirectly)either a delay element or oscillator which produces the regeneratedsignal.

To determine the correct error vector, the DPD identifies which sourceevent corresponds to which regenerated event (because of the feedbackloop configuration, the regenerated event is also referred to as afeedback event). Due to latency and loss of events (either by design ordue to data loss), particularly in the source path, the DPD shouldaccount for these effects when determining correspondence between sourceand feedback events.

Stability of PLLs (including DPLLs) can be achieved by updating theerror vector frequently. Interpolation of lost (or late) source data isone technique which can keep the error vector current. If the timingreferences between the source and receiver are not zero aligned to eachother, source events and feedback events will have an arbitrary averageoffset. Because the offset is arbitrary, it is not possible for therecovered timing to necessarily share the same phase as the source. Theoffset (which can be estimated from the initial difference) should berecorded and nulled from all subsequent error vectors. Even with sharedzero alignment between devices, the accuracy of the zero alignment willdirectly affect the quality of the regenerated phase. Note that evenwhen zero alignment is present, the receiver may choose to null anyoffset.

FIG. 25 is a schematic diagram of an electronic system 820 according toanother embodiment. The electronic system 820 includes source ICs 811 a,811 b, . . . 811 m and destination ICs 812 a, 812 b, . . . 812 n, whichare electrically connected to one another via a digital interface 813.

The source ICs 811 a, 811 b, . . . 811 m receive signals SIG1, SIG2, . .. SIGm, respectively. Although an example with three source ICs isshown, and number of source ICs can be included (for instance, onesource IC, two source ICs, or three or more source ICs). Furthermore,although each source IC is shown as receiving one signal fordistribution over the digital interface 813, one or more of the sourceICs can distribute multiple signals.

As shown in FIG. 25, the source ICs 811 a, 811 b, . . . 811 m eachreceive a common reference signal (COMMON REF). The source ICs 811 a,811 b, . . . 81 m also receive separate system reference signals SYSTEMREFs1, SYSTEM REFs2 . . . . SYSTEM REFsm, respectively. Although anexample with separate system reference signals is shown, one or more ofthe source ICs 811 a, 811 b, . . . 811 m can share a system referencesignal. Furthermore, in certain implementations, the common referencesignal is used as the system reference signal for of one or more of thesource ICs.

The source ICs 811 a, 811 b, . . . 811 m operate to generate digitalrepresentations of the timing of the signals SIG1, SIG2, . . . SIGmusing the common reference signal and the local system reference signalsSYSTEM REFs1, SYSTEM REFs2, . . . SYSTEM REFsm, respectively. Thedigital timing representations of the signals SIG1, SIG2, . . . SIGm areprovided over the digital interface 813 to the destination ICs 812 a,812 b, . . . 812 n.

The digital interface 813 can be implemented in a wide variety of ways.In one example, the digital interface 813 is an Ethernet interface. Inanother example, the digital interface 813 is an I²C, SPI, or otherserial interface. Although various examples of the digital interface 813have been provided, any suitable digital interface can be used,including both standardized and custom interfaces.

The destination ICs 812 a, 812 b, . . . 812 n receive the digitalrepresentations of the timing of the signals SIG1, SIG2, . . . SIGm, viathe digital interface 813. Although an example with three destinationICs is shown, and number of destination ICs can be included (forinstance, one destination IC, two destination ICs, or three or moredestination ICs).

As shown in FIG. 25, the destination ICs 812 a, 812 b, . . . 812 n eachreceive the common reference signal (COMMON REF) that is also common tothe source ICs 811 a, 811 b, . . . 811 m. The destination ICs 812 a, 812b, . . . 812 n also receive separate system reference signals SYSTEMREFd1, SYSTEM REFd2, . . . SYSTEM REFdn, respectively. Although anexample with separate system reference signals is shown, one or more ofthe destination ICs 812 a, 812 b, . . . 812 n can share a systemreference signal. Furthermore, in certain implementations, the commonreference signal is used as the system reference signal for of one ormore of the destination ICs.

The destination ICs 812 a, 812 b, . . . 812 n operate to recover one ormore of the signals SIG1, SIG2, . . . SIGm based on the received digitaltiming representations, the common reference signal, and the localsystem reference signals SYSTEM REFd1, SYSTEM REFd2, . . . SYSTEM REFdn,respectively. Both frequency and phase of the signals SIG1, SIG2, . . .SIGm can be recovered.

Although each destination IC is shown as recovering each of the signalsSIG1, SIG2, . . . SIGm, any combination of signals can be recovered asdesired.

The electronic system 820 can be used to provide precise distribution ofsignals (including, but not limited to, clock signals) to multipledestination ICs. For example, in certain applications, tens or hundredsof clock signals can be digitally communicated to a large number of ICsusing a digital bus.

FIG. 26A is a schematic diagram of a source device 850 according to oneembodiment. The source device 850 includes a TDC 841, a formatconversion circuit 842, a synchronization circuit 843, an LO 844, and anupconvert circuit 845.

The upconvert circuit 845 provides frequency upconversion to a localoscillator signal received from the LO 844 to generate an upconvertedclock signal for the synchronization circuit 843. The synchronizationcircuit 843 compares the upconverted clock signal to the common clocksignal to control synchronization of the TDC 841 and the formatconversion circuit 842.

The TDC 841 generates digital time stamps representing transition timesof the input signal. The digital time stamps are processed by the formatconversion circuit 842, which aids in converting the digital time stampsto a format suitable for a common interpretation of time stamps acrossmultiple distributed destination devices.

FIG. 26B is a schematic diagram of a destination device 860 according toone embodiment. The destination device 860 includes a format conversioncircuit 851, DPLL 852, a synchronization circuit 853, an LO 854, and anupconvert circuit 855.

The upconvert circuit 855 provides frequency upconversion to a localoscillator signal received from the LO 854 to generate an upconvertedclock signal for the synchronization circuit 853. The synchronizationcircuit 853 compares the upconverted clock signal to the common clocksignal to control synchronization of the format conversion circuit 851and DPLL 852.

The format conversion circuit 851 interprets a time reference point ofthe received digital time stamps, and converts the digital time stampsto a format suitable for processing by the DPLL 852. The DPLL 852processes the digital time stamps to recover the signal.

In certain implementations, the DPLL 852 recovers both the frequency andphase of the signal received by a corresponding source device. When sucha signal is distributed to multiple destination devices (for instance,tens or hundreds of destination devices) each destination device canrecover the original signal with precise frequency and accurate phase.

FIG. 27A is a schematic diagram of a source IC 870 according to oneembodiment. The source IC 870 includes a TDC 861, a format conversioncircuit 862, a synchronization circuit 863, and a system PLL 865, whichserves as an upconvert circuit. The source IC 870 includes pins forreceiving a signal (SIG), a system reference signal (SYSTEM REF), and acommon reference signal (COMMON REF), and for sending the digital timingrepresentations of the signal.

The system PLL 865 generates a system clock signal for thesynchronization circuit 863. The synchronization circuit 863 comparesthe system clock signal to the common reference signal to controlsynchronization of the TDC 861 and the format conversion circuit 862.

FIG. 27B is a schematic diagram of a destination IC 880 according to oneembodiment. The destination IC 880 includes a format conversion circuit871, a DPLL 872, a synchronization circuit 873, and a system PLL 875,which serves as an upconvert circuit. The destination IC 880 includespins for receiving digital timing representations from a source IC (forinstance, from the source IC 870 of FIG. 27A), a system reference signal(SYSTEM REF), and a common reference signal (COMMON REF), which iscommon to the source IC. The digital timing representations are receivedfrom the source IC over a digital interface.

The system PLL 875 generates a system clock signal for thesynchronization circuit 873. The synchronization circuit 873 comparesthe system clock signal to the common reference signal to controlsynchronization of the format conversion circuit 871 and the DPLL 872.As shown in FIG. 27B, the DPLL 872 recovers the signal, includingfrequency and phase information. The signal can be used locally and/ordistributed off-chip.

FIG. 28 is a schematic diagram of another embodiment of a clocksynchronization and frequency translation IC 890. The IC 890 illustratedone example of an IC that can be used as either a source IC ordestination IC, thereby enhancing flexibility. For example, a firstinstantiation of the IC 890 can be used as a source IC and a secondinstantiation of the IC 890 can be used as a destination IC.

The IC 890 of FIG. 28 is similar to the IC 40 of FIG. 1, except that theIC 890 further includes a synchronization circuit 881, a source formatconversion circuit 882, and a destination format conversion circuit 883.

When operating as the source IC, time stamps of an input signal can begenerated in a variety of ways, such as by using any of TDCs 4 a-4 dand/or auxiliary TDCs 22. The source format conversion circuit 882formats the time stamps to generate data suitable for transmissionoff-chip in a variety of ways, such as via the serial port, themultifunction pins (M PINS), and/or separate pins. Additionally, thesystem clock PLL 13 serves as an upconvert circuit for a systemreference signal received on the system reference pins (XOA, XOB). Incertain implementations, the synchronization circuit 881 provides sourcesynchronization. However, other configurations are possible. Forexample, in another implementation, the synchronization circuit 881 isomitted in favor of providing synchronization using the system clockcompensation circuit 16 operating in the closed loop configuration shownin FIG. 10 in combination with the auxiliary NCOs 21.

When operating as the destination IC, digital timing representations canbe received via the IC's pins, for instance using the serial port, thestatus and control pins 23 interface (for instance, by way of auniversity asynchronous receiver-transmitter or UART), and/or suitabledigital interface. The digital timing representations are processed bythe destination formation conversion circuit 883, and thereafterprovided to a DPLL (for instance DPLL 6 a, DPLL 6 b, and/or a dedicatedDPLL) to recover the signal. Additionally, the system clock PLL 13serves as an upconvert circuit for a system reference signal received onthe system reference pins (XOA, XOB), and the synchronization circuit881 can be used to provide synchronization.

Extrapolation of Timing Events for Enhanced PLL Update Rate

A PLL generates an output signal (also referred to as a generated signalor synthesized signal), which is provided by feedback to the PLL's phasedetector. The phase detector compares the feedback signal to a referencesignal to generate a phase error signal used for controlling the PLL'sloop and generation of the output signal. At the PLL's phase detector,the feedback signal is the coherent with the reference signal.

The feedback signal and/or the reference signal can be decimated (forinstance, by integer frequency division) prior to reaching the phasedetector. The pre-decimated signals are coherent frequency multiples,and thus are coherent once per cycle of the respective decimated signal.Additionally, the reference signal and the generated signal are alsocoherent multiples, though their periodicity corresponds to the leastcommon multiple (LCM) of the two decimation ratios after common divisorsare removed.

A PLL can decimate or divide signals for a variety reasons. In a firstexample, decimation is used to provide frequency translation. Forinstance, since coherence of the reference signal and the feedbacksignal is enforced at the phase detector, decimation permits thegenerated signal frequency to be a rational multiple of the referenceinput frequency. In a second example, decimation is used to operatecomponents of the PLL within their operating frequency ranges.

Clock signals can be characterized by the timing of one of the edgeevents (rising or falling), designated as phase 0 (zero), and thus theirphase is observable periodically. Thus, a PLL operates as a sampledcontrol system.

PLLs often operate at a relatively low update rate to preserve valuabletiming information. However, operation at higher update rates can havebenefits to certain performance metrics of the PLL. For example, whenclocks are decimated, the timing information present on the skippedevents is lost, depriving the PLL of information.

In certain embodiments herein, a PLL is implemented to retain the timingfrom some or all of the lost timing events arising from decimation.

Implementing the PLL in this manner can enhance certain performancemetrics of the PLL. For example, in the reference signal path, it allowshigher oversampling relative to the control loop bandwidth and thusbetter filtering of certain types of phase jitter. Furthermore, in boththe reference signal path and the feedback signal path, operating with ahigher rate can provide a quicker indication of shifts in frequencies.For example, operating with higher update rate in the reference pathprovides the PLL's loop with better tracking capability, while operatinghigher update rate in the feedback path permits wider control loopbandwidths and consequently faster acquisition.

In certain configurations, a reference signal of the PLL has a carrierfrequency and an embedded subcarrier frequency. Providing a referencesignal with an embedded subcarrier can provide a number of advantages.For example, the carrier frequency can convey desired frequencyinformation while the subcarrier frequency can convey desired phaseinformation. Additionally, the PLL recovers timing events associatedwith the sub-carrier, and processes the timing events to extrapolatetiming events at a frequency greater than the subcarrier frequency.

In applications in which the subset of events selected by decimation isnon-arbitrary, such as when using a reference signal with an embeddedsub-carrier, intermediate stages of decimation can be used to meet themaximum operating rates of PLL components. When the timing of thenon-arbitrary subset is conveyed to the phase detector, the PLL willalign to these events. For example, one or more decimations stages canbe synchronized to pass special events associated with a sub-carrierthat provides phase information.

FIG. 29 is a schematically depicts various timing event sequences forone example of intermediate decimation. A first signal 1001, a secondsignal 1002, and a third signal 1003 are depicted. In one embodiment,the first signal 1001 represents an input reference signal that includesan embedded sub-carrier including phase information, the second signal1002 represents the input reference signal after an intermediatedecimation, and the third signal 1003 represents the embeddedsub-carrier after final decimation of the first signal 1009. AlthoughFIG. 29 illustrates an intermediate decimation by 3 and a totaldecimation by 9, any suitable values of decimation can be used.

Various timing event sequences are shown in FIG. 29, including asequence {S_(j,k)} of timing events resulting from intermediatedecimation, and a sequence {S_(j,0)} of timing events resulting fromfinal decimation. The entire sequence {S_(j,k)} represents a coherentfrequency multiple.

As shown in FIG. 29, there is a regular relationship between thesequence {S_(j,k)} of timing events and the sequence {S_(j,0)} of timingevents. In particular, the sequence {S_(j,0)} is a sub-set of the entiresequence {S_(j,k)}.

Accordingly, any event from the superset or full sequence {S_(j,k)} canbe used to estimate one or more elements from the subset or sub-sequence{S_(j,0)}. For example, using an estimated and/or ideal periodicity ofthe superset events, ΔT, the value of S_(j,0) can be approximated asS_(j,k)−k·ΔT.

In certain implementations. AT can be estimated from the sequenceitself, such as by using a time-variant estimation that does notadversely impact the PLL's dynamic response.

Within the implementation of a DPLL, each element of this sequence canbe represented by a digital time stamp indicating the timing of theevent. Accordingly, in certain implementations, the sequence {S_(j,k)}of timing events is digitally represented using time stamps from a TDC.

FIG. 30A illustrates one example of a backward extrapolation of asequence of timing events. As shown in FIG. 30A, a signal 1011 isillustrated along with a decimated signal 1012 corresponding to thesignal 1011 divided by a factor of four. Although decimation by 4 isillustrated, any suitable values of decimation can be used.

As shown in FIG. 30A, the timing events of the sequence {S_(j,k)} arenot ideally spaced. Rather, the timing events include timing informationε₀, ε₁, . . . ε₂ indicating phase jitter and/or instantaneous frequencyof the signal 1011.

Various timing events have been extrapolated to relative to an edge ofthe decimated signal 1012, corresponding to timing information atS_(j,0). Additionally, a first extrapolated timing event 2021 has beenused to estimate the value of S_(j,0) as S_(j,1)−1·ΔT, a secondextrapolated timing event 2022 has been used to estimate the value ofS_(j,0) as S_(j,2)−2·ΔT, and a third extrapolated timing event 2023 hasbeen used to estimate the value of S_(j,0) as S_(j,3)−3·ΔT.

The extrapolated timing events 2021-2023 include timing information ε₀,ε₁, . . . ε₂ indicating phase jitter and/or instantaneous frequency ofthe signal 1011. If the goal were to accurately estimate that specificevent S_(j,0), the timing information ε₀, ε₁, . . . ε₂ may not beuseful.

However, providing information about the phase jitter and/orinstantaneous frequency of the signal 1011 to a phase detector of a PLLallows the phase detector to measure coherence of the reference andgenerated signal sequences at the lowest rate of timing, but withinformation that is updated at a higher rate.

FIG. 30B illustrates one example of forward and backward extrapolationof a sequence of timing events. FIG. 30B is similar to FIG. 30A, exceptthat FIG. 30B illustrates forward extrapolation of the thirdextrapolated timing event 2023 to the event S_(j,1) rather than tobackward extrapolation to the event S_(j,1).

Timing events can be extrapolated in a wide variety of ways, including,but not limited to backward extrapolation, forward extrapolation, or acombination thereof.

FIG. 31 is a schematic diagram of a DPLL 1060 according to anotherembodiment. The DPLL 1060 of FIG. 31 is similar to the DPLL 50 of FIG.2A, except that the DPLL 1060 of FIG. 31 includes an input divider 1050and a digital phase detector 1051. The digital phase detector 1051includes an extrapolation circuit 1052, which generates one or moreextrapolated timing events to enhance the operation of the DPLL 1060.The extrapolated timing events can be for the reference signal and/orfeedback signal.

In one embodiment, the input signal 1055 can include a carrier frequency(for instance, 10 MHz) and an embedded subcarrier frequency (forinstance, 1 kHz). Additionally, the carrier frequency provides frequencyinformation and the subcarrier frequency provides phase information.Although the input divider 1050 could have a division value selected torecover only the subcarrier (for instance, R=10,000 for a 10 MHz carrierand 1 kHz sub-carrier), running the DPLL with a relatively low updaterate can provide poor performance.

In contrast, the illustrated DPLL 1060 includes the extrapolationcircuit 1052 for generating extrapolated timing events. For example, theextrapolated events can include extrapolations of timing events of thecarrier frequency used to estimate the sub-carrier events. Since theextrapolated timing events include phase jitter and/or instantaneousfrequency information, the operation of the DPLL 1060 is enhanced. Theextrapolated timing events can be for the reference signal and/orfeedback signal.

For example, providing information about the phase jitter and/orinstantaneous frequency of the signal 1055 to the phase detector 1051allows measurement of coherence of the signal sequences at a desired lowrate of timing, but with information that is updated at a higher rate.

FIG. 32 is a schematic diagram of a DPLL 1070 according to anotherembodiment. The DPLL 1070 of FIG. 32 is similar to the DPLL 80 of FIG.3, except that the DPLL1070 of FIG. 32 includes a digital phase detector1071 including an extrapolation circuit 1071. The extrapolation circuit1071 generates one or more extrapolated timing events in accordance withthe teachings herein. The extrapolated timing events can be for thereference signal and/or feedback signal.

FIG. 33 is a schematic diagram of another implementation of frequencytranslation loops 1150 for a clock synchronization and frequencytranslation IC. The frequency translation loops 1150 of FIG. 33 aresimilar to the frequency translation loops 150 of FIG. 5, except thatFIG. 33 illustrates an embodiment including a DPLL 1106 that includes atime stamp processor 1131 implemented with an extrapolation circuit1132. The extrapolation circuit 1132 generates one or more extrapolatedtiming events in accordance with the teachings herein. The extrapolatedtiming events can be for the reference signal and/or feedback signal.

Fast Locking PLLs for Low Loop Bandwidth

A locking time of certain PLLs can be relatively long. For example, azero-delay PLL with low loop bandwidth and a low frequency referencesignal can have a prohibitively long locking time.

A PLL's locking time is based on a transient response of the closed loopnegative feedback system of the PLL. For example, the duration of thelocking transient can depend on local oscillator frequency offsetrelative to the reference frequency, initial phase offset present at thephase detector, and low pass filter parameters (for instance, bandwidthand phase margin).

In certain implementations herein, a PLL is locked in multiple steps,including an initial frequency acquisition step in which the PLL isoperated open loop. By executing an algorithm to separate and correctfor individual components of the PLL in a suitable order, the durationof the locking transient can be reduced or minimized.

Frequency is the derivative of phase. In certain implementations, afrequency offset between the reference input clock signal and the PLL'slocal oscillator (i.e. feedback clock signal) is minimized by afrequency offset correction. In certain implementations, the frequencyoffset correction is executed without inclusion of an initial phaseoffset parameter.

A DPLL can provide suitable processing for facilitating implementationof such an algorithm.

For example, an adjustment to a digital phase detector (DPD) is oneexample of a suitable mechanism for correcting phase offset. Forinstance, the initial phase offset can be quantified and subtracted fromDPD outputs to the loop filter. Implementing the adjustment in thismanner provides a number of advantages, such as injecting only theresidual phase offsets generated by the frequency mismatch between DPDinputs. Once the DPLL achieves steady state, the loop filter output isstored in memory for use as the initial loop filter output in thesubsequent algorithm step.

In implementations in which the DPLL implements feedback frequencytuning via a numerically controlled oscillator (NCO), consecutive phasemeasurements of each DPD input can be differentiated and compared.Additionally, the result of the comparison can be used to calculate thefractional frequency error (for instance, normalize frequency error) ofthe feedback source relative to the reference input clock. Thecalculated fractional frequency error can then scale the currentoscillator control value (including application of a control valueversus frequency linearization transfer function) to produce a frequencycorrection value normalized to the NCO control word. Updating the activeNCO control value with the summation of the previous control word andthe frequency correction value provides relatively low instantaneousfrequency offset of the DPLL's NCO output, yielding an initial feedbackfrequency for the subsequent algorithm step.

Performing the application of the correction factor to the active NCOcontrol value in multiple steps with magnitude determined based on aprogrammable limit and the time duration between consecutive updatesallows for the implementation of a controlled rate of change of thefrequency transition. Implementing the DPLL in this manner providesenhanced performance for systems in which the output clock of the NCO isused externally to the device.

Once the frequency error has been reduced or minimized, the phase offsetbetween the DPD's inputs is approximately constant and an offsetcorrection may be implemented.

In certain implementations, phase correction is provided by physicallysynchronizing the dividers between the highest intermediate frequencysynchronous to the DPLL's local oscillator and the DPD's feedback inputusing the DPD's reference input signal as the synchronization source.This can result in the first edge out of the synchronized dividers beingphase aligned to the DPD's reference input signal within 1 UI of thefrequency at the input of the first affected divider.

When it is desirable to limit the frequency deviation incurred as afunction of the phase offset, the phase offset may be quantified andtranslated to a representation as the integration of the frequencydeviation limit over a calculable time duration. This frequency offsetcan then be negated and applied to the feedback source for thecalculated time duration to achieve the desired phase alignment withoutexceeding the frequency deviation limits.

Once the frequency and phase offsets between DPLL reference and feedbacksources have been reduced or minimized, the DPLL is operated in closedloop operation to compensate for any error in the calculation of theaforementioned correction factors.

To minimize the duration of this correction stage, a bandwidth reductionalgorithm can be implemented to begin loop acquisition at a much largerloop bandwidth and progressively decay versus time to a final operatingbandwidth for steady state operation and adherence to systemspecifications.

FIG. 34 is a method 1210 of phase and frequency locking according to oneembodiment. The method 1210 can be implemented, for example, using anysuitable PLL described herein.

The method starts at step 1201, where a frequency offset between areference signal and a feedback signal of a PLL is detected. In oneembodiment, the method is implemented in the clock synchronization andfrequency translation IC 40 of FIG. 1.

The frequency offset can be detected in a variety of ways, includingopen loop or closed loop detection using any suitable frequency offsetdetection circuit. In one example, reference monitors (for instance, thereference monitors 18 of FIG. 1) are used to detect the frequencydifference. In another example, the frequency offset is detected bysubtracting an initial phase offset from an output of a digital phasedetector, and detecting the frequency offset based on a residual phaseoffset of the digital phase detector. In yet another example, aderivative of successive phase measurements of the reference clocksignal is compared to a derivate of successive phase measurements of thefeedback clock signal.

The method 1210 continues to step 1202, in which the frequency offset ofthe PLL is compensated by providing an open loop frequency offsetcorrection. Thus, the feedback loop of the PLL is opened or broken whenprovided the frequency offset correction. The frequency offset of thePLL can be compensated in a wide variety of ways. In one example, a loopfilter output value is controlled to provide compensation. In anotherexample, an NCO is adjusted based on normalizing a fractional frequencyerror by a control word of an NCO, and updating the NCO based on thenormalized frequency error (for instance, updating the active NCOcontrol value with the summation of the previous control word and thefrequency correction value).

The PLL's loop can be opened and closed in a wide variety of ways, suchas by using a loop controller (for instance, the loop controller 85 ofFIG. 3). In certain implementations, the loop controller controls and/orcoordinates the operations of step 1202.

In certain implementations, the frequency offset is gradually providedto limit change to an output frequency of the PLL.

The method continues to a step 1203, in which a phase offset between thereference signal and the feedback signal is compensated by providing aphase offset correction after the frequency offset correction. The phaseoffset correction can be provided in a variety of ways. In one example,a feedback divider of the PLL is synchronizing based on timing of thereference clock signal. For instance, a PLL's dividers can be physicallysynchronized to a highest intermediate frequency synchronous to thePLL's local oscillator and the phase detector's feedback input using thereference input signal as the synchronization source. Such a phasealignment can result in the first edge out of the synchronized dividersbeing phase aligned to the reference input signal within 1 UI of thefrequency at the input of the first affected divider.

In certain implementations, the phase offset is compensated by graduallyproviding phase adjustment to limit an output frequency deviation of thePLL.

In certain implementations, the loop controller controls and/orcoordinates the operations of step 1203.

In an ensuing step 1204, a residual error of the PLL is compensated bylocking the feedback signal to the reference signal with the feedbackloop of the PLL. Thus, the PLL's feedback loop is closed when correctingthe residual error. In certain implementations, the loop bandwidth ofthe PLL is changed over time to enhance locking speed. For instance, abandwidth reduction algorithm may be implemented to begin loopacquisition at a much larger loop bandwidth and progressively decayversus time to a final operating bandwidth for steady state operationand adherence to system specifications. The loop bandwidth can bechanged in a variety of ways, such as by programming different numericcoefficients of a digital loop filter (for example, FIG. 2A).

In certain implementations, the loop controller controls and/orcoordinates the operations of step 1204.

FIGS. 35A-35E illustrate various embodiments of DPLL circuitry for phaseand frequency locking.

FIG. 35A illustrates a portion of a DPLL including a digital phasedetector 51 and a subtraction circuit 1211. As shown in FIG. 35A, aninitial phase offset is subtracted from the output of the digital phasedetector 51.

FIG. 35B illustrates a portion of a DPLL including a memory 59 and adigital loop filter 52. In certain implementations, a frequency offsetcorrection is provided by loading a loop filter output value from thememory 59.

FIG. 35C illustrates a portion of a DPLL including a differentiationcircuit 1231, a digital phase detector 51, a digital loop filter 52, andan NCO 53. In certain implementations, a frequency offset is detected bycomparing a derivative of successive phase measurements of the referenceclock signal to a derivate of successive phase measurements of thefeedback clock signal. Additionally, a fractional frequency error iscalculated based on the comparison, and the NCO adjusted based on thefractional frequency error. For example, fractional frequency error canbe normalized to a control word of an NCO, and updated based on thenormalized frequency error.

FIG. 35D illustrates a portion of a DPLL including a digital phasedetector 51, a feedback divider 54, and a synchronization circuit 1241.In certain implementations, a phase offset is corrected by synchronizingthe feedback divider 54 based on timing of the reference signal.

FIG. 35E illustrates a portion of a DPLL including a digital phasedetector 1251. The digital phase detector 1251 includes a slew ratelimiter 1252 for limiting a slew rate of the DPLL. In certainimplementations, one or more components of a PLL operates with a slewrate limit to prevent sudden changes to the output clock signal.

Phase Shift Detection

In many applications, a phase locked loop (PLL) is deployed purely forfrequency synchronization and the initial steady-state phase alignmentbetween input timing reference and output clock is of no concern to thesystem's operation and therefore arbitrary.

Due to practical system operation, the source of a timing reference maybe switched to a redundant, frequency synchronous source with arbitraryphase relationship to the original reference. In the event that such aswitch occurs outside of the context of the PLL control logic, the phasedifference between timing references is introduced to the PLL phasedetector (PD) as phase error and results in an undesired, transientfrequency deviation on the output clock.

Knowledge of the characteristics of such events allows for animplementation of appropriate detection circuitry to trigger properhandling of such phase shifts, preserving desired system operation.

It is often the case that sufficiently small phase shifts can result intransient effects which degrade system performance and, as a result, themagnitude of the phase shifts that are desirable to detect andcompensate (i.e. detection threshold T_(R)) is on the order of or lowerthan the peak to peak noise of the timing reference itself.

In such a case, simply observing the phase error of each the timinginput and comparing it to the detection threshold can result in thedetection of false positives leading to the decimation of the timingreference's phase information.

One primary characteristic of a phase shift is that it has a non-zeromean, while the timing noise, assumed to be relatively Gaussian, is zeromean. Therefore, summing N consecutive samples will gain up the phaseerror contributed by a phase shift by a scalar of N, but not the timingnoise contributors effectively increasing the signal to noise ratio ofthe detection circuitry.

This allows for the use of an extended detection threshold, T_(E), asdetermined by: T_(E)=N×T_(R) while still detecting a phase shift ofmagnitude T_(R) without resulting in false detections.

FIGS. 36A-36D are graphs of various example of phase step detection.

It should be noted that in lieu of using an extended threshold, theinstantaneous phase error input may be differentiated prior to thewindowed accumulation and compared against the original detectionthreshold. This would provide substantially the same increase in signalto noise ratio in the detection circuitry.

For further noise immunity, a majority rule processing is applied a setof N samples of the detection circuitry output can be used. Thisprovides two distinct benefits.

First, by using a set of detector outputs versus a single positiveoutput provides increased noise immunity against a false positive beingdeclared in the event that a single phase error sample contained anexcessively large amount of noise. As Gaussian noise is technicallyunbounded, the peak to peak noise grows with sample size, and systemsare requested to run ad infinitum by customers, this is a valuableimprovement. However, when a single phase error value is substantiallylarge, a single shot measurement approach, with an independent detectionthreshold well in excess of the noise, can still be of value inconjunction with this invention.

Second, if a phase shift exactly equal to the detection thresholdoccurs, then it is likely that noise contributors will cause somedetection decisions to yield a negative result while the mean phaseshift measured is still greater than or equal to the detectionthreshold. If all N samples of the detection circuitry output arerequired to yield a positive result then minimum detectable the phaseshift magnitude will be equal to the detection threshold plus the inputnoise of the timing reference.

Once the detection output yields positive results, but not enoughsubsequent samples have been collected to populate the N sample setentirely with post-shift sample which is required to detect a phaseshift via majority rule, the output of the PD may be suppressed toprevent the PLL from responding to a potential phase shift. If afterfull population of the N sample set with post-shift samples, a majorityrule vote does not confirm the potential step, the suppressed samplesmay be re-introduced downstream of the phase shift detector.

In one embodiment, a reference switching circuit (for instance, thereference switching circuit 19) is implemented in accordance with one ormore of the features discussed above. In another embodiment, a referencemonitor (for instance, the reference monitors 18 of FIG. 1, thereference monitor 602 of FIGS. 21 and 22, and/or the reference monitor670 of FIG. 23) is implemented in accordance with one or more of thefeatures discussed above.

FIG. 37A is a schematic diagram of one embodiment of a phase shiftdetector 1301. The phase shift detector 1301 detects a phase shift of areference clock signal (REF CLOCK) based on timing of a system clocksignal (SYSTEM CLOCK). The phase shift detector 1301 receives adetection threshold T_(R). In certain implementations, the detectionthreshold T_(R) is received from a user over an interface, such as aserial port.

In the illustrated embodiment, the phase shift detector 1301 operateswith an extended detection threshold 1302. In certain implementations,the phase shift detector 1301 observes phase shift over N cycles of thereference clock signal. For instance, the phase shift detector 1301 canaccumulate the detected phase shift over the N cycles, therebycalculating a windowed average. For example, summing N consecutivesamples will gain up the phase error contributed by a phase shift by ascalar of N, but not the timing noise contributors effectivelyincreasing the signal to noise ratio of the detection circuitry. Thus, aphase shift of magnitude T_(R) can be detected without resulting infalse detections.

FIG. 37B is a schematic diagram of another embodiment of a phase shiftdetector 1310. The phase shift detector 1310 includes a phase errordifferentiation circuit 1311 and a windowed accumulator 1312.

Additionally or alternatively to an extended threshold, a phase shiftdetector can differentiate instantaneous phase error prior to windowedaccumulation, and the result can be compared against the originaldetection threshold T_(R).

FIG. 37C is a schematic diagram of another embodiment of a phase shiftdetector 1320. The phase shift detector 1320 includes a majority ruleprocessing circuit 1321, which is applied to a set of N samples of thereference clock signal taken by phase shift detector 1310.

The majority rule processing circuit 1321 increases noise immunityagainst a false positive being declared in the event that a single phaseerror sample contains an excessively large amount of noise.Additionally, when a phase shift substantially equal to the detectionthreshold occurs, then it is likely that noise contributors will causesome detection decisions to yield a negative result while the mean phaseshift measured is still greater than or equal to the detectionthreshold. If all N samples of the detection circuitry output arerequired to yield a positive result, then minimum detectable the phaseshift magnitude will be equal to the detection threshold plus the inputnoise of the timing reference.

Once the detection output yields positive results, but not enoughsubsequent samples have been collected to populate the N sample setentirely with post-shift sample to thereby detect a phase shift viamajority rule, the output of the PD may be suppressed to prevent the PLLfrom responding to a potential phase shift. If after full population ofthe N sample set with post-shift samples, a majority rule vote does notconfirm the potential step, the suppressed samples may be re-introduceddownstream of the phase shift detector.

FIG. 37D is a schematic diagram of another embodiment of a phase shiftdetector 1330. The phase shift detector 1330 operates based on timestamps from a TDC 1331. The phase shift detector 1330 can include one ormore of the features discussed above.

Reduction of Buildout Clock Switching Residue

Phase buildout clock switching can be used to reduce or minimize theoutput phase deviation resulting from the acquisition of a new referenceclock by compensating for a phase difference equal to an estimate of theaverage offset.

In certain implementations herein, apparatus and methods for improvingthe quality of this estimate and thus reduce the residual phase errorcaused by the switch are provided.

Assuming the average phase difference in a phase lock loop (PLL) isconstant over time (that is, the reference and output frequencies arenominally equal), the average of the first N phase error samplesprovides a better estimate of the offset as N increases. The exact rateof improvement varies with the statistical distribution of samples andthe type of averaging performed (for instance, uniform or weighted).

The value of N cannot be arbitrarily large. Either the loop is inactiveduring sample collection, which delays the intended operation; or theloop is active and begins to react to phase errors, which affects thesample measurements. Furthermore, if the nominal frequencies are notequal, the phase error measurements will record a linear trendproportional to the frequency difference.

Restricting the maximum value of N such that the collection period ismuch less than the loop's time-constant can minimize the interactionbetween the phase averaging and the PLL operation. By temporarilyincreasing the loop's time-constant (reducing the bandwidth), themaximum value of N may be increased while limiting the interaction.

When there is an offset in frequencies, a deterministic time dependentphase offset is accumulated. The effect of this error can be mitigatedby limiting N such that the error contributions in the average offsetare dominated by random effects. Alternately, the linear trend may becanceled from the resulting average. The slope of the trend-line may beextracted from the samples themselves, or by some other estimate of thefrequency offset.

Further variants to the phase offset data collection can be employed.Rather than using a fixed value of N, the noise and trend-line of thedata may be examined, as it becomes available, to determine whether thecollection period should be ended or extended. Also, adjustments made bythe PLL to its output are known and this knowledge may be used to cancelthe effects of these adjustments from the phase offset measurement.

A multi-sample average can be a better estimate for the offset than asingle-sample. Although there are limitations to the amount ofimprovement possible, the significant error sources associated with theaverage may be limited or otherwise mitigated.

In certain embodiments herein, a deviation resulting from theacquisition of a new reference clock is compensated for a phasedifference equal to an estimate of the average offset taken using amulti-sample average. In certain implementations, the estimate isobtained by comparing time stamps from TDCs (for example, the TDCs 4 a-4d of FIG. 1). Additionally, rather than comparing one time stamp toanother, the difference between multiple pairs of corresponding timestamps are calculated and averaged.

FIG. 38 is a schematic diagram of a phase offset detection system 1400according to one embodiment. The phase offset detection system 1400includes a first TDC 1401, a second TDC 1402, a multiplexer 1403, a PLL1404, and a phase offset detector 1405.

The phase offset detector 1405 detects a phase offset between the firstreference clock signal (REF1) and the second reference clock signal(REF2) based on multiple samples of each clock signal. For example, amulti-sample average can be a better estimate for the offset than asingle-sample. The phase offset detector 1405 can include one or morefeatures discussed above. In certain implementations, the multiplexer1403 is controlled by a reference switching circuit, such as thereference switching circuit 19 of FIG. 1. For example, the referenceswitching circuit can control which reference clock signal is providedto a phase and/or frequency detector of the PLL 1404 to serve as atiming reference.

As shown in FIG. 38, the phase offset detected by phase offset detector1405 can be used to compensate the PLL 1404. For example, the detectedphase offset can be used to reduce or minimize the output phasedeviation resulting from the acquisition of a new reference clock bycompensating for a phase difference equal to an estimate of the averageoffset. The phase offset detector 1405 can provide phase adjustmentusing the detected phase offset in any suitable manner.

The phase offset detection schemes described above can be incorporatedinto any of the PLLs described herein. For example, in one embodimentthe clock synchronization and frequency translation IC 40 of FIG. 1 isimplemented with one or more features of phase offset detectiondiscussed above.

Aligning to Phase Information Lost in Decimation

In a zero-delay phase lock loop (PLL), it is often desirable toinitialize the reference and output (feedback) clocks such that theyexhibit minimal or relatively low initial skew (phase offset). Thislimits the phase error that the PLL pull-ins upon activation and reducesor minimizes the duration and magnitude of the phase/frequency transientresponse.

When decimation (such as by frequency division) of these clocks occur,the pair of edges which are best aligned may not be included in the setof edges available to the phase detector (PD). Knowledge of thedecimation ratio for each clock signal can allow the device toextrapolate the timing of the best edge pairing, thus minimizing theinitial phase error.

Zero-delay operation of a PLL describes the steady-state operation ofthe loop such that the reference and output clocks align—at least on asubset of the clock events. An extension to zero-delay, hitlessoperation, also defines that the loop acquisition transient isminimized.

When the clocks are decimated, to achieve frequency translation and/ormeet the maximum frequency limits of the PLL components, the process ofachieving hitless operation can be complicated. For a DPLL, it ispossible for the PD to operate upon timestamps representing thereference and output signals. Calculations upon these timestamps providea mechanism to address various challenges, including those that arisefrom implementation of hitless mode.

Given a timestamp, S_(j), the interval between timestamps in a path,T≈S_(j+1)−S_(j), and the decimation ratio associated with that path, M,all the events between S_(j) and S_(j+1), which were lost, can beestimated. Denote the non-decimated sequence of events starting withS_(j) as S_(j,k), where S_(j)=S_(j,0) and k is in [0 . . . M−1]. ThenS_(j,k) can be approximated by S_(j,k)≈S_(j)+T·k÷M.

FIG. 39 is a graph of one example of possible phases after a divide bythree.

FIG. 40 is a graph of one example of time stamp interpolations.

If the DPLL is configured such that the output frequency is an integermultiple of the reference frequency, every reference event has anexactly corresponding output event. In this case, interpolation appliedto the output events will yield the nearest match.

In certain implementations, the time distance (D) to the nearest matchis constrained to D≤0.5 UI, where the unit interval, UI=T÷M. For anyreference event with timestamp X, there are consecutive decimated outputevents represented by S_(j) and S_(j+1) such that S_(j)≤X<S_(j+1). Letthe divided phase of X relative to S_(j) be denoted asφ=(X−S_(j))÷(S_(j+1)−S_(j)). Then the extrapolated output index whichmatches closest is K=round(M·φ). If K=M, S_(j+1.0) is the nearest match,otherwise S_(j,K) is the best. Recall S_(j,K)≈S_(j)+T·K÷M, so applyingan offset of T·K÷M to the sequence of output timestamps will cause theDPLL to align to the pair of edges which match best, even when K≠0, thatis, the alignment pair is obscured by the output decimation.

Similarly, if the reference frequency is an integer multiple of theoutput frequency, every output event has an exact correspondingreference event. As before, interpolation will yield the best matchingpair, only now the extrapolation is performed upon the referencesequence.

In certain embodiments herein, one or more of the features discussedabove is implemented in a digital phase detector (for example, thedigital phase detector 51 of FIG. 3) and/or a time stamp process (forexample, the time stamp processor 131 of FIG. 5).

FIG. 41 is a schematic diagram of a DPLL 1660 according to anotherembodiment. The DPLL 1660 of FIG. 41 is similar to the DPLL 50 of FIG.2A, except that the DPLL 1660 of FIG. 41 includes an input divider 1650and a digital phase detector 1651. The input divider 1650 receives aninput reference 1655. The digital phase detector 1651 includes aninterpolation circuit 1652, which is implemented in accordance with oneor more features discussed above. For example, the interpolation circuit1652 can account for lost edges arising from decimation of the divider1650 and/or the divider 54.

Implementing the DPLL 1660 with the interpolation circuit 1652 canprovide a number of advantages. For example, knowledge of the decimationratio for each clock signal can allow the device to extrapolate thetiming of the best edge pairing, thus minimizing the initial phaseerror.

FIG. 42 is a schematic diagram of a DPLL 1670 according to anotherembodiment. The DPLL 1670 of FIG. 42 is similar to the DPLL 80 of FIG.3, except that the DPLL 1670 of FIG. 42 includes a digital phasedetector 1671 including an interpolation circuit 1671, which isimplemented in accordance with one or more features discussed above.

Applications

Devices employing the above described schemes can be implemented intovarious electronic devices. Examples of electronic devices include, butare not limited to, consumer electronic products, parts of the consumerelectronic products, electronic test equipment, communicationinfrastructure, etc. For instance, one or more clock synchronization andfrequency translation ICs can be used in a wide range of analog,mixed-signal, and RF systems, including, but not limited to, dataconverters, chip-to-chip communication systems, clock and data recoverysystems, base stations, mobile devices (for instance, smartphones orhandsets), laptop computers, tablets, and wearable electronics. A widerange of consumer electronics products can also include such ICs forInternet of Things (IOT) applications. For instance, one or more clocksynchronization and frequency translation ICs can be included in anautomobile, a camcorder, a camera, a digital camera, a portable memorychip, a washer, a dryer, a washer/dryer, a copier, a facsimile machine,a scanner, a multi-functional peripheral device, or a wide range ofother consumer electronics products. Furthermore, electronic devices caninclude unfinished products, including those for industrial, medical andautomotive applications.

In one example, a clock synchronization and frequency translation ICprovides jitter cleanup and synchronization in GPS, PTP (IEEE-1588),and/or SyncE applications. In a second example, a clock synchronizationand frequency translation IC is included in a base station (forinstance, a femtocell or picocell) to control clocking for baseband andradio. In a third example, a clock synchronization and frequencytranslation IC controls mapping/demapping for a transport network, suchas an optical transport network (OTN), while providing jitter cleaning.In a fourth example, a clock synchronization and frequency translationIC provides holdover, jitter cleanup, and phase transient control forStratum 2, 3e, and 3 applications. In a fifth example, a clocksynchronization and frequency translation IC provides support for dataconversion clocking, such as analog-to-digital (A/D) and/ordigital-to-analog (D/A) conversion, for instance, for JESD204B support.In a sixth example, a clock synchronization and frequency translation ICprovides timing for wired infrastructure support, such as cableinfrastructure and/or carrier Ethernet.

CONCLUSION

The foregoing description may refer to elements or features as being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/feature is directlyor indirectly connected to another element/feature, and not necessarilymechanically. Likewise, unless expressly stated otherwise, “coupled”means that one element/feature is directly or indirectly coupled toanother element/feature, and not necessarily mechanically. Thus,although the various schematics shown in the figures depict examplearrangements of elements and components, additional interveningelements, devices, features, or components may be present in an actualembodiment (assuming that the functionality of the depicted circuits isnot adversely affected).

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel apparatus, methods, andsystems described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein may be made without departingfrom the spirit of the disclosure. For example, while the disclosedembodiments are presented in a given arrangement, alternativeembodiments may perform similar functionalities with differentcomponents and/or circuit topologies, and some elements may be deleted,moved, added, subdivided, combined, and/or modified. Each of theseelements may be implemented in a variety of different ways. Any suitablecombination of the elements and acts of the various embodimentsdescribed above can be combined to provide further embodiments.Accordingly, the scope of the present invention is defined only byreference to the appended claims.

Although the claims presented here are in single dependency format forfiling at the USPTO, it is to be understood that any claim may depend onany preceding claim of the same type except when that is clearly nottechnically feasible.

What is claimed is:
 1. An integrated circuit (IC) with referencemonitoring, the IC comprising: a clock measurement circuit configured togenerate a plurality of digital measurements of a reference clock signalbased on timing of a system clock signal; and a reference monitorconfigured to generate a monitor output signal indicating whether thereference clock signal is within a tolerance of one or more toleranceparameters, wherein the reference monitor comprises a statisticalprocessing circuit configured to process the plurality of digitalmeasurements to generate an estimate of measurement uncertainty, and tocontrol a latency of the reference monitor in generating the monitoroutput signal based on the estimate of measurement uncertainty, whereinthe latency corresponds to a measurement duration of the referencemonitor in determining whether the reference clock signal is within thetolerance.
 2. The IC of claim 1, wherein the statistical processingcircuit is configured to compute a variance of the plurality of digitalmeasurements over a time window.
 3. The IC of claim 2, wherein the oneor more tolerance parameters comprises a nominal period and a periodoffset limit, wherein the statistical processing circuit is furtherconfigured to control the latency based on comparing the variance to theperiod offset limit.
 4. The IC of claim 1, wherein the statisticalprocessing circuit is further configured to determine a number ofsamples of the reference clock signal sufficient to estimate a period ofthe reference clock signal within a confidence interval.
 5. The IC ofclaim 1, wherein the one or more tolerance parameters comprises a jitterlimit.
 6. The IC of claim 1, wherein the statistical processing circuitis further configured to generate a plurality of estimates ofmeasurement uncertainty associated with a plurality of partiallyoverlapping time windows.
 7. The IC of claim 1, wherein the statisticalprocessing circuit is configured to compute a mean and a variance of theplurality of digital measurements over a time window.
 8. The IC of claim1, wherein the clock measurement circuit comprises a time-to-digitalconverter (TDC) configured to generate a plurality of digital timestamps representing a plurality of transition times of the referenceclock signal.
 9. The IC of claim 8, further comprising adigital-phase-locked loop (DPLL) configured to process the plurality ofdigital time stamps.
 10. A method of reference monitoring in a clocksystem, the method comprising: generating a plurality of digitalmeasurements of a reference clock signal based on timing of a systemclock signal; processing the plurality of digital measurements togenerate an estimate of measurement uncertainty using a referencemonitor, wherein processing the plurality of digital measurementscomprises computing a variance of the plurality of digital measurementsover a time window; and controlling a measurement latency of thereference monitor based on the estimate of measurement uncertainty. 11.The method of claim 10, further comprising detecting whether thereference clock signal is within a tolerance of one or more toleranceparameters using the reference monitor.
 12. The method of claim 10,wherein processing the plurality of digital measurements comprisesdetermining a number of samples of the reference clock signal sufficientto estimate a period of the reference clock signal within a confidenceinterval.
 13. The method of claim 10, wherein generating the pluralityof digital measurements comprises generating a plurality of digital timestamps representing a plurality of transition times of the referenceclock signal.
 14. The method of claim 13, further comprising processingthe plurality of digital time stamps using a digital-phase-locked loop(DPLL).
 15. A reference signal monitoring system with dynamicallycontrolled latency, the reference signal monitoring system comprising: atime-to-digital converter (TDC) configured to generate a plurality ofdigital time stamps representing a plurality of transition times of areference clock signal; and a reference monitor configured to generate amonitor output signal indicating a status of the reference clock signal,wherein the reference monitor is configured to process the plurality ofdigital time stamps to generate an estimate of measurement uncertainty,wherein the reference monitor is further configured to generate theestimate of measurement uncertainty based on computing a variance of theplurality of digital time stamps over a time window, and to control alatency of the reference monitor in generating the monitor output signalbased on the estimate of measurement uncertainty.
 16. The referencesignal monitoring system of claim 15, wherein the reference monitor isfurther configured to control the latency based on comparing thevariance to a period offset limit.
 17. The reference signal monitoringsystem of claim 15, wherein the reference monitor is further configuredto determine a number of samples of the reference clock signalsufficient to estimate a period of the reference clock signal within aconfidence interval.
 18. The reference signal monitoring system of claim15, wherein the monitor output signal indicates whether the referenceclock signal is within a jitter limit.
 19. The reference signalmonitoring system of claim 15, wherein the reference monitor is furtherconfigured to generate a plurality of estimates of measurementuncertainty associated with a plurality of partially overlapping timewindows.
 20. The reference signal monitoring system of claim 15, furthercomprising a digital-phase-locked loop (DPLL) configured to process theplurality of digital time stamps.